aren't there keys that let you unlock the version exclusive areas by trading?
And yeah, by the time of SuMo they were really playing around with the whole "alternate timelines" thing. I just think it's kind of stupid. Like.. to me it makes everything feel way more... it feels less like they're trying to build a world and more like they're just winging it. It makes me feel like they don't care about building a coherent world or story, and I just wish they did. Like, I would be much more satisfied if like... the protagonists in the two versions were canonically different people who existed in the same world and that's why they go different places and experience different stuff. And you explain away the version differences like "different pokemon spawns or a different box legendary" as just game mechanics and not a literal fact of the universe because every damn copy of the game is it's own unique timeline in cannon.
Like, I know this is the way they currently do it, but wouldn't you like it better if they did it differently?
Technically, it all is alternate timelines. Like the remakes, 3rd versions, and reimagining's all take place in separate, but very similar timelines, and that's been the case for a while. The modern games played up to it, which gave reasonings as to why the remake and 3rd version games are different than the mainline dual versions. And really, the best, and really only, way to put it is that they're all just alternate timelines/dimensions, they just fleshed it out starting with Gen 7. When it comes to referencing back, they tend to use the 3rd versions as the definitive one.
Like some examples on this:
Emerald is the game that SuMo used when bringing back Anabel. Since Emerald never got a remake, the Battle Frontier wasn't a thing, and thus Anabel was missing, they had to give an explanation on how she came back, especially since it follows ORAS's timeline.
Platinum is the game Legends used when it came to being a prequel. All the Sinnoh backstory correlates to Platinum exclusive story.