No I am not claiming that the Church should get credit for literally everything, just that credit should not be denied for the things the Church actually did, especially when the early Christians you're ascribing that credit too were also part of the Church.
Understood and formally declared are two different things. Would you deny the Founding Fathers of America credit for the Declaration of Independence simply because the principles withing were understood for centuries beforehand? Self evident truths, as they put it it. Regardless of the prior understanding, which we can at best speculate over, there is great significance to the formal declaration of something by the proper authority.
When I say that this was broadly understood, I don't mean that these early Christian writers I mentioned had published lists of Church canon or that they were collaborating on what was and wasn't scripture. I mean literally that they just understood these works to be authoritative, we can infer which ones they did not from any list they published but just from seeing which works they quoted. Ireaneus for example quoting outed 21 out of 27 New Testament books during his lifetime, Origen likewise references 25 of them in a single work, not in an attempted to show what was canon, but simply to list how they built on each other.
Theres this misconception Atheists love to push. They'll say stuff like, "Actually the Bible was codified at the Council of Nicea in 325 AD [it's always Nicea for some reason], prior to that there was a lot of debate about what was and wasn't scripture with the council rejecting books that made Jesus look less divine. There are several dozen known first and second century Gospels which were rejected, despite being well regarded, simply because they proved Jesus wasn't God!" And this is quite far from the truth, Biblical canon was established by popular consenus very early on, certainly by the end of the second century most Christians understood which works were authorative. As a matter of fact as early as the 1st Century you can see Christians starting to reference our modern Christian canon with Pope Clement (96 AD) referening 1st and 2nd Corinthians in his own epistle to the Church at Corinth and Papias (100 AD) refrencing the creations of the Gospel of Matthew and Mark.
I think the reason you see this line of rhetoric a lot is because sola scriptura proponents tend to argue by citing chapter and verse (often without regard for context) and act as though the Bible as we know it today is all they ever needed. This notion is disrespectful to the hundreds of years of dedication and development that it took to go from the original scriptures all the way to a formally recognized printed book that has been divided into chapter and verse.
In my experience it's less from Protestants misusing Bible verses (though this definently happens) and more from Catholics and Orthodox justifying why they engage in practices not in the Bible. For example, if a sola scriptura guy is talking to a Catholic and the Catholic starts takling about praying to saints or purgatory and the Protestant says, "Chapter and Verse please, show me where the term 'Pope' or 'Mass' are in the Bible!" the Catholic will usually say, "You see the Church was first, we gave you the Bible, before our councils solidified things people were running around unsure of what was real and what wasn't. And since we created the canon we reserve the right to retroactively modify it to add certain works of Church fathers and Papal decrees to it. Not everything divine is in the Bible".
So in a sense, you get this weird situation where both hardcore Atheists and devoute Catholics and Orthodox will push the same myth. The Atheist will push this myth because Biblical canon was being decided by a council hundreds of years after the fact it cheapens the quality of the canon and scirpture, since in their version of things a few works were singled out for portraying Jesus was great and divine while works of seemingly equal value in the early Christian community were rejected for disproving Christianity or making Jesus seem less than God. Then you'll have Catholics and Orthodox push the same myth that they are solely responsible for deciding Biblican canon at some council so this gives them the authority to add to the scirptures, defeating Sola Scriptura. It's a weird kind of horse shoe thing were two seemingly opposite sides need to push the same narrative but for vastily different reasons.
The majority of people back then were illiterate. That isn't to say being illiterate makes you unintelligent, but if they can't read the scriptures they have to rely on someone who can. Furthermore the simple fact that there were numerous incidents of widespread heresies in those days is proof enough that people could be mislead by those who either do not fully understand the scriptures or wish to misuse them for their own gain.
That's right, there were many (several dozen in fact) aprochrapyl books you'd probably view as heretical that we know existed from back in the day, and many more we can infer existed but we have no records of, and while some were hoodwinked by them, these were defeated in the court of public opinion, lambasted by Christian writers, and fell into obscurity. Remeber, in an era when every single copy has to be handwritten people aren't going to waste days of work rewritting something most people regarded as BS, it's like you alluded to, everything has to be handwritten, everything has to be carried by hand, people knew Jesus's story either because they knew him persaonlly or through some second or third hand source after the fact. They could quickly suss out what was BS and what wasn't, that's why the three synotpic Gospels which tell roughly the same story survived while random Gospels that have Jesus not being crucified or saying or doing random things were quickly disregarded. Additionally works by respected writters were preserved because early Christians recognized the importance of these men. For example if you're living in 100 AD and you get a copy of 1st Corinthians you might be like, "Woah this was written by Paul! I read about him in the Book of Acts, I should take his words seriously.