I think people expected it to be BG3, engine or not engine, 100 years in the future or not. And if it wasn't going to be a true sequel, why call it BG3? I would guess marketing mostly. I really think that Larian gained enough renown with the RPG community that if they would just said that they are making a D&D 5e game people would be more than happy to buy and play it. It's like if Mass Effect 3 was a 1st person shooter 100 years in the future with different plot and massive visual redisign of everything. It wasn't really Mass Effect 3 right?