it appears Sonic Generations is going for the "bundle" approach, meaning you're still getting the original game with the remaster
(EDIT: correction, the tweet wording means availability in bundles, not "bundled in." that's a bit worse.)
and not losing it if already owned. personally would've preferred separate listings as they're now locking it behind a bundle purchase but it's... better, by accessible preservation standards. certainly an improvement over the Origins kerfuffle; at least it's not straight-up gone, nor causing a FOMO rush.
why you don't just, get the original as a freebie like several other steam remaster releases do, ive no clue. sega's like that.