They promoted their end to end encryption promise for messages, so I think they would've promoted it again if it still were true for the video call feature.
edit:
Google, for example, promised their video calls to be encrypted with Duo. https://blog.google/products/duo/meet-google-duo-simple-1-to...
Is it, if it's WebRTC, only in-transit encryption or would that mean that the calls content isn't visible for WhatsApp/Facebook at all?
However, it's still possible to put a server between two participants so that instead of communicating directly, they would be communicating through this intermediate endpoint which would have access to call content - without call participants even knowing. So ultimately it's a question of trusting a particular service, not the protocol used.