Curb your enthusiasm, the 400 pokemon in the main dex was already confirmed.

The extra 35 are the Kanto starters and a bunch of legendaries from various generations. They are CONFIRMED to not be in the base game so they are in the data for one of two reasons:

1. They were originally going to be in the game and were part of the earlier batches of Pokemon that were ported over. Since they're all starters and legendaries those are very possibly higher priority 'mons that would be ported over before others in the conversion process.

2. They will be released later as event exclusive 'mons for tournaments or Pokemon Center events. Not out of the question for Game Freak but it would be a bit odd if that was the case since we're still pretty certain they can't be ported over from other sources.

Of note there are also 2 empty slots that were also found. These are likely for yet-to-be-released mythical pokemon like a Galarian Mew or whatever.

35 of approximately 409 missing mons.

Assuming this isn't all just scrap data like with all the RS items in FRLG.