Yeah, a decent litmus test is if you sign into a service from a new device and without much effort all your chats/messages/history or whatever is there, the security is weak.

I got off WhatsApp years ago so I am not sure what's changed but back then if you signed on from any random browser, it was able to sync everything instantly and you'd see all your messages. This was after they claimed that it was E2E encrypted. What was explained to me at the time was that you share your encryption key with Facebook and hence the syncing.