It was a few years ago when I read Signal's statement about this, so I'm afraid I don't have a link for you.
I believe you when you say Molly functions, but it's important to note that without Signal's blessing, anyone using Molly can be locked out of the network (and their chats and contacts) at any moment. It's not the same as official interoperability.
I wonder if the Digital Markets Act will eventually force it.
A game category made up by people who think "boomer" means "anyone more than a few years older than me".