WhatsApp does use the Signal protocol, but unlike Signal only applies it to Messages, Calls and Status.
Your profile info, who you're talking to, when you're using the service, groups you're in, channels you're following and much more is left unprotected intentionally.
For instance, Signal sends your profile end-to-end-encrypted instead of leaving it freely accessible on servers.
yeah it's quite disappointing, I wish Matrix saw some more (individual!) funding
(organisations that have funded Matrix in the past generally want new features, but not core work on Matrix to make the experience any better 🥴)