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what is so hard to understand with soft-reboots/retcons?bThere is one timeline which is the one that was tweeted.
"BUT ORAS AND..."
It was a tongue in cheek reference to the original games, it doesn't change the timeline or imply that it's a split timeline now. Obviously new stuff will be introduced each gen to get you interested and it's introduced as "new" or "just discovered" this happened all the way back in Gen 2 and Gen 3 we have all these new Pokemon and it takes place at the same time as Gen 1. Sure in the remakes they no longer say this since every "new" discovery is retconned to the past because they cant remake a game with less features than the current version. Who would've bought a remake if you could only still do the same stuff of the original?
If you want to get autistic about it then there are dozens of timelines since each version tells a slightly different story, which version is canon? X or Y? OR or AS? The third version is the canon version? Where did you pull that shit from your ass? That has never been stated you just assume since it's the newest version it's the canon version well the remakes are the newest versions. Hell the tweet doesn't mention FRLG it mentions Red and Green does that mean FRLG was never canon? it mentions RS not Emerald does that mean third versions are not in continuity? Who was the canon gym leader in in the last gym of BW? etc.
If you want to make 2 or 3 timelines then you have to go all out and make dozens of timelines, one where Blue is canon with gold another where Blue is canon with Silver, another where Red is canon with Gold and another where Red is Canon with Sil since different canon Pokemon appear in each version and you can do that for each fucking game release. and you get hundreds of timeline possibilities with just the games released.
TLDR Dont be autistic about the timeline and let it flow new functions will be retconned into existing before the game they were introduced.