Yeah, it seems a bit hard at times to reconcile the existences of these regions in previous games which likely actually aren't mentioned because they hadn't been come up with yet, especially in gen 1, with Kanto being right next to Johto yet no one in Kanto mentioning it, or its having an American character (Surge) who never mentions Unova or whatever adjacent region he was presumably from. Then again, munna appears to be alluded to in gen 1, which may be a case of the developers finding ways to turn off-hand comments into foreshadowing, which kind of helps.
I don't remember much about whether the text of the 1st gen games insinuates that the gen 1 pokédex is an exhaustive list or not (the anime pretty clearly states it isn't right from the get-go, what with the ho-oh showing up in the first episode). Oak is, however, claimed to be the "world authority on all pokémon", which makes it suspect that he'd call the gen 1 dex complete when it doesn't even include every pokémon from Kanto (some pokémon that live there, such as the gen 2 nocturnal pokémon found in Kanto, aren't part of it). Again, I guess some of that can be chalked up to the developers not yet knowing whether the game would be a one-off game or not, and thus not insinuating there would be more to discover before it was clear there would be. And, as I said before, this does force retroactive suspension of disbelief by leaving earlier games a bit difficult to reconcile with later ones.
And it does seem occasionally that the developers will take the easy way out of this, having introduced multiverse theory around gen 6, which is a pretty quick fix for any plot holes, as you could say any inconsistencies are due to different games taking place in different parallel universes and whatnot. What exactly this supposed gen-1-specific universe would look like still may raise further questions, though, as it seems the only way to explain its having a world pokémon authority only knowing about 150 pokémon without living in an isolated region (which, again, Surge's presence rules out) would be mass extinctions of all other pokémon (with the exception of a few possibly surviving species rendered cryptids such as munna) with the outside regions ending up in a similar state to Orre, being only populated by introduced species, or not having wild pokémon at all. Which means that such band-aid hand-waves leave the situation of some of these gens rather bleak under the surface.
Though I guess that isn't all that new, considering enforced bleakness has occurred in cases such as gen 2's version of Kanto having had a lot of places seem to undergo hardships due to the lack of memory space leaving it impossible to fit them (Safari Zone comes to mind).