iMessage encryption is proprietary and not E2EE with most iCloud setups, WhatsApp‘s is literally the defacto industry standard (Whisper / Signal Protocol) without Meta holding your keys for you.
WhatsApp was already owned by Facebook when they started working on E2EE with the Signal Organization (literally the Signal devs who worked on WhatsApp‘s E2EE). There have also been independent audits that verify the claims.
But then again, it‘s more fun to spread FUD and rock the „but metadata isn‘t“ yadda yadda train I guess.