They are using the same encryption as Signal and it was done by the people running and developing Signal.
"As of today, the integration is fully complete. Users running the most recent versions of WhatsApp on any platform now get full end-to-end encryption for every message they send and every WhatsApp call they make when communicating with each other."
The founders have lamented that the project is screwed, so you're left with a claim made my Zuckerberg and his buddies?