Is your source for “what privacy experts say” a sad jpeg meme, really?
Also, no matter what some distracted expert might say, the only fact that matters is that none of Signal’s marketing claims are verifiable: the feature you are referring to happens server-side. Nobody but Signal knows what runs server-side. The guarantee of “not knowing who’s talking to whom” isn’t built into the protocol itself. This is where trust enters the picture.

The dominant paradigm in cybersecurity is that trust is not proof of anything. Math is. And “sealed senders” isn’t that.