One could argue that, but if you did you are wrong.
The game already has the Non-Galarian forms of pokemon with Galarian forms as well as the pokemon with Alolan forms programmed in ... this includes Kantonian Slowpoke.
And no, we have no idea what is counted in the over 100 returning ... it is all assumption, which is my point ... we don't know.
The following are assumptions that are being made:
1) That these pokemon here: https://www.serebii.net/swordshield/transferonly.shtml
Are all included as part of the over 100 returning in the respective DLCs. If these pokemon can be transferred in when Home arrives, then they likely aren't including as part of the "over 100 returning."
2) That slowpoke counts as part of the over 100.
So we actually do not know what exactly the "realistic best" would be, or how unlikely the number is. They advertised DP as having over 100 new pokemon was 107. They didn't show much gameplay of the expansions, which to me means they are still being worked on. There are people counting Mew as one of the returning expansion pokemon which makes no sense as Mew was advertised to be in the base game before release.