D:OS fans are the core target audience the game is being made for and not for anyone who is not a D:OS fan.
The DOS fanbase and those who are not fans of DOS are not mutually exclusive. Larian can be making the game for both the huge (and current) crowd of DOS fans as well as those who never played DOS1&2 (i.e those who are not DOS fans).
Yes all fanbases are to one extent or another cross-cutting. But don't you think it is telling that the only fanbase that is near-universally thrilled with this game is the D:OS fanbase, whereas the BG and D&D fanbases are at a minimum divided and at most only lukewarm in their reactions to this game? I mean if, as Swen and other Larian people have been saying, this game is not a D:OS game and is truly a BG and D&D game, then shouldn't at least some portion of the D:OS fanbase feel like this is NOT their game? I love the PoE games. But I don't care at all for The Outer Worlds. So just because I love some games of a certain developer does not mean I will or should love all their games, right? But I cannot find any D:OS fans who intend to pass on BG3. That says something to me. It says a whole heck of a lot, actually, that the D:OS fanbase itself fully expects BG3 to be a D:OS game but just with D&D rules and set in the Realms.