This has already been confirmed a non story. He said this around the launch time and Larian have since confirmed that the section has been found and played.

This article shows the poor state of gaming "journalism". Stories taken from each other and no actual work done by the "journalist". So false rumours spread.

Kotaku have an updated version of the story, link below. I have no idea if they broke the story first or are another of the copycats though.
https://kotaku.com/baldurs-gate-3-astarion-voice-actor-hidden-area-section-1850860934

Maybe he's gone method and this is just Astarion fucking with us 🤔

I will be sad if the voice actor is misinformed and it's just cut content.
Edit: The content being referred to has been found a while ago, see other response

Extended Astarion sex scene? I'm down

It's Astarion. He fuckin' lying!

That whole bit didn't seem that hidden to me, just seemed natural to go ahead and do it to cross another thing off the list.

I kinda wondered about something like this the other day playing D4 (yeah yeah, I know, I'm getting BG3 when this season's over). Not even thinking as big as a whole section of the game, but, like, is there still a very well-hidden (non-randomized) chest or statue or something that no one has interacted with yet? If so, I wonder where it is. If not, I wonder how long it took for the very last whatever to be found and clicked on. An hour? Day? Week?