While I have played RPGs (including BG1) before, it was BG2 that started my love for computer RPGs.
So, I played a lot of it and after an adjustment-phase (which I always need) I did enjoy BG3 immensely.
But to answer your question, it completely depends on what Baldurs Gate is for you.
BG3 is, in my opinion, one of the possible ..evolutions of BG2.
It has not really that much in common with BG1, at least not directly.
So, where it the characters and their personality - through biographies, selection sounds and banter system - BG to you? Then, yes it is a true sequel.
Do you remember the way battles where designed and fought when you hear "Baldurs Gate"? Then BG3 is different, since both rules and battle system differ.
Do you think Baldurs Gate is the story of the Bhaalspawns? This is harder, but I would say it is not a sequel on face value.
The issues with story connections are that we do not know them yet, since we are pretty in the dark about the story.
I, myself, loathed the battles in BG2, liked the story and loved the npcs. Especially the romance (something new for me). I also think Aerie is, personality wise, underrated and the best of the four original romances - and I will die on that hill!
Anyways, this is the reason that, for me, this is the game I have waited 20 years for. But others find it is spitting on Baldurs Gate legacy.
I do, however, wonder if those who prefer BG1 over BG2 like BG3 less - since it is further removed from it. But I am not sure there is a correlation.
