I know the OP didn't mention about a platform being dependent on having a mobile phone number...but an advantage of matrix is that it does not require such a number. I state this because the OP focused on desktop, so i am making an assumption here. Also, matrix (and other federated-based platforms) need not be hosted in a single silo...much like email, can be hosted by numerous servers, and yet interact freely.
I've never heard of it being peer reviewed by Signal.
And even if it is, there's a lot of stuff around Whatsapp apart from the protocol that most likely is not.
It was Open Whisper Systems. My bad.
They're the people behind Signal, kind of assumed you meant them :-)