Yes it is, they actually use the signal protocol,[0] but they collect metadata which Signal supposedly doesn't (you can't really know)

[0] https://en.wikipedia.org/wiki/Signal_Protocol#:~:text=Severa...

https://www.reddit.com/r/privacy/comments/v7tsou/is_whatsapp...

It seems to be but there is more to it than that.

I thought they recorded the metadata - who talks to who and when. (For the uninitiated, that is as valuable or more valuable than the message contents.)