So, I think that in spirit it's still the successor of Baldur's Gate 1 and 2. Because there are so many things that people who did play and like Baldur's Gate 1 and 2 will still recognise in the new one. It's still about your party. It's still about big personalities clashing with each other and relationships. It's still a party-based game, you still need to do combat, you will recognise a lot of D&D rules - even if you haven't played D&D in 20 years. You will still recognise all the spells, et cetera. So, to me it's a true sequel, but we are bringing it into the 21st century by saying, "Look, it's glorious 3D."
So, Larian's "BG3" is a sequel because it's a party-based RPG with colourful character and with combat in it - and the combat uses a D&D ruleset. He didn't even dare add that Baldur's Gate has specifically RTwP combat - because, of course, Larian's "BG3" doesn't.
There are loads of games that fit Walgrave's description that aren't called Baldur's Gate series games, and there are thousands that fit the description if not counting the D&D ruleset qualifier.
Walgrave's claim is the equivalent of saying that any first-person game where you play as a single character and use a variety of weapons to shoot at lots of things is a DOOM series game or a Half-Life series game. What Walgrave is saying is that there is no similarity between Larian's D&D game and the Baldur's Gate series and so he couldn't think of something that actually justifies calling Larian's "BG3" a Baldur's Gate series game.
Hey, our RPG is nearly as all RPGs with new rules. That's why we call it Baldur's Gate 3!!