If I remember this was talked about in here before and it was revealed he's using someone else's Minecraft RPG engine with minor tweaks and name changes.
A bit ago Blockgame (his MC "mmo" server) had some kind of issue related to sieges and player-built cities and ultimately he couldn't fix the problem, so he opted to swap the plugin with another, neither of which he made himself, and he talked about swapping them on stream as well. you can essentially climb through his blockgame dev streams to find out that ini editing (as others have mentioned) indeed are the limit to his capabilities as if he really was an amazing programmer, there'd be no need to pester the original plugin author with fixing the issue in question, nor would be need to swap the plugin, because he'd have just simply fixed it by altering the plugin itself directly, or writing a different one. IIRC this issue went on for at least a week before swapping for a different player town system plugin.
They also made the games open source, that’s the biggest reason. Both of them have source ports that add new features and allow the games to run on modern systems without much hassle. Releasing official tools also helped a ton.
But the reality of the situation is that people will mod anything and everything if they like it. People mod capcom and Nintendo games despite both being hostile to modders. Minecraft has the biggest modding community there is despite mojang basically ignoring them and constantly breaking their stuff.
Why the fuck would anyone want to spend their time modding into Heartbound though is anyone’s guess, since you could probably remake the entirety of heartbound from scratch in a similar amount of time.
True on the OSS part, that was a big part, and those games are also really good to boot, that one part always gives mods a high probability of happening even if it's ridiculously annoying to mod, like Diablo 2. Heartbound isn't at that level of quality, it could be, but it would require Jason to get some help or start working on it like he was having an autistic fit of hyperfocus unless he wants to spend the next 6 years making Heartbound.
I have a theory that someone with 0 knowledge in programming, art or music could remake Heartbound as it is now by 3 years time if they just started making the game and built the skills as they went but they also spent an autistic amount of time doing so. If that person were to finish Heartbound according to 100% of Jason's promises for the game's future, you could probably do it in 10 total at most since there really isn't a lot of combat. I know probably 0.001% programming, nothing on making music, and a bit on art but I can see such a thing being doable, whether one would actually do it is the operative part.
To be fair, it's not really as if Jason's coding knowledge is overwhelmingly above the layperson, the ost is probably the toughest part to make, but you can just probably save up 500 bucks and throw it at one of the many musicians on the net and the graphical/art part of it would either just be sheer volume of practice and passion for it, or finding an artist to do it for you. To do it they'd just need patience, the willingness to improve, and the drive to create something fun in the end.
What is there in Heartbound right now is not a whole lot, it's a bit unfortunate but he doesn't exactly have a whole lot to show for Heartbound being in development for over 8 years and at the current pace, anyone who ever wishes to see the game complete would probably be better off hoping for someone to just dev-cuck Jason by finishing it for him, otherwise I don't really see a finished Heartbound happening at least not anytime soon.
But that's my .02 on Heartbound and its future. Shartbound could also be a great game if someone committed to making it as such.