>>28829694GSC makes it pretty clear that dark and Steel type were only recently classified, not that they didn't exist until a week before the game, they just were classed as different types.
As for the 100 new Pokemon, they never say anything about when they were discovered, only that they are new species. There's a 3 year gap in which the new monsters begin migrating to Kanto. You can assume that the Johto Pokemon always existed, just only in the Johto region before then.
Well, why weren't they in Oak's Pokedex? Because at the start of gen 1, he's only just invented it. Remember that he gives you the dex in Gen 2 as well, it's almost like he was upgrading it to class new Pokemon from other regions.
The remakes drop all of those plot points however, and FRLG even has Oak saying that new Pokemon were found in the Sevii Islands, the likes of which had never been seen in the Kanto region before upgrading you to the National Dex.
