Well yes and no. You can compile a client yourself but you can't use it with the official server, you have to use the binaries they put out for that. So there's a bit of a gap there.

their clients are open source

Their clients are but they sell proprietary licenses, so when they proclaim that There is no WhatsApp 'backdoor', you have to take their word for it.

I'm aware they lag behind in updates, only reason I chose to use Molly was because if I recall correctly they use Google Firebase for notifications, and I use grapheneOS and don't have Google play service installed. So not sure if I would get notifications in my setup if I used signal witn no play services.

Correct me if I'm wrong though