That is what damon lindelof and carlton cuse have said. But its very questionable if its true. Neither have ever been trustworthy in terms of their accounts of what happened and what was planned. Season 3 was plagued with declining viewership rates and audience fatigue.
By the later parts of season 3, the ratings had fallen by about half of what they were in season 2;. Massive budget cuts were necessary to continue the show and it was decided to cut the number of episodes to retain the quality of the show but still right-size the budget to the much diminished audience for the show. They were kind of telling the truth, but not really telling the truth. So rather than tell the truth of a show in decline and budget cuts, they told a story about how they were trying to make a better show for the fans.
I dont think before the end of season 3 that the show ever had a plan of where it was going. The individual seasons had been planned, but no more than that. It was a typical JJ Abrams mystery box show and the thing about his mystery boxes is that they inevitably turn out to be empty.
Also, alot of the good ideas in the early seasons came from Jack Bender. The later seasons (4-6) are more the "pure" ideas of damon lindelof and carlton cuse.
You got details wrong and then some.
The time travel shit was explicitly stated at SDCC around season two, though they said it jokingly because they didn't want to explicitly state "yeah, we are doing time travel shit" and spoil the plot. This is covered in detail in "The Revolution Was Televised"; a book about "Second Golden Age of TV" shows from the 00s/early 10s when they discuss Lost.
Season three's low ratings was due to Heroes stealing their thunder as the "anti-Lost" and the fact that the show runners and ABC started feuding over the creative direction and ABC fucking with the scheduling to such a degree that they got told they were going to only air SIX episodes in the Fall and then take the show off the air for three months and not bring it back until February next year, where the remaining 16 episodes would air with no skip week.
Because of this, the producers were forced to basically BURN those six episodes, since they couldn't do much since the show was going to be MIA for three months so any momentum from them would be lost due to the hiatus. They basically made this meta-textual even, with the plot line about the main characters taken prisoner by the Others stuck in zoo cages for those episodes and wasting one episode on two new characters who were killed off at the end of said episode.
As such, the gap between S3 E1-6 and 7-22 was filled with a LOT of behind the scenes fighting with the network as ABC started threatening cancellation and the show runners being livid and claiming ABC was trying to sabotage the show (which was being renewed season by season) due to it no longer being the media darling because of Heroes. In the end, the solution was to give the showrunners what they wanted: three guaranteed seasons that would air no matter what, and a reduced episode count so they could stop with the filler shit once and for all and basically put the writers on notice of "we have X-number of episodes to finish the story divided among three seasons so every episode going forward has to count". Though the exact episode count for each season got reworked by the 2008 writer strike, where they reworked the episode count so that while they were able to seemlessly cram in season 4 into the episodes they ended up with, season five got an extra episode to counterbalance it.
The one thing that DID get changed mid-series was the flashbacks being retired and replaced with flash forward timeline. This was explicitly done by the writers against their original plans for the show to center on the island, as a splashy holy shit moment to get fans talking and the media to go "LOST IS BACK BABY!!!". Which worked out, as it revitalized the show and got everyone back talking about it, while Heroes never fucking recovered from the writers strike fucking it over.
