No it's not. Repeating it doesn't turn it into a fact.
It uses D&D rules, displays D20s and has D&D lore and therefore can't be a D:OS3. You might say that to emphasize your point but insisting on this comes over as rather silly.
Do some things look like Larian took them from the Original Sin games? Yes. And it would be an economically bad choice not to build on their successful foundations. Does that mean that this turns BG3 into D:OS 3? Please...
I didn't like D:OS I and II and get different vibes from the pre-alpha gameplay of BG3. It doesn't feel like the old Baldur's Gate games for sure - but it feels like a D&D game. Especially with the awesome cinematics in between which ooze D&D.
I would argue that it's impossible to give the look & feel of BGI and II and at the same time cater to a big enough audience to not drive this game into a very narrow niche (aka BG grognards). There have to be compromises - hugh ones - to not only please old BG players but also win new players for the BG and D&D franchise.
