I am still interested to know the details of how they came to this decision. Why Signal instead of Matrix.

AFAIK, signal doesn’t federate, There is no “signal server-to-server” protocol. When people say “The Signal Protocol”, they are talking about a cryptographic protocol, not a network protocol.

As for why they wouldn’t use Matrix, I would assume it’s just too heavy of a protocol for the scale they operate at. IIRC, Matrix isn’t just a chat protocol. It’s a multi-peer cryptographic state synchronization protocol. Chat is (was?) just the first “easy” application they were going to apply it to. (Now I’m curious if they still have plans for that at some point.) They’ve been making great strides in improving the efficiency, at least in the client-server API (I haven’t been paying attention to the server-server API at all), but it’s still going to be a heck of a lot more compute heavy than whatever custom API they’re providing.