https://signal.org/blog/whatsapp-complete/

"A year ago [2015], we announced a partnership with WhatsApp and committed to integrating the Signal Protocol into their product, moving towards full end-to-end encryption for all of their users by default."

Now obviously things may have drifted since 2016 but it definitely sounds like - at some point - WhatsApp was based on the Signal protocol.

"Over the past year, we’ve been progressively rolling out Signal Protocol support for all WhatsApp communication across all WhatsApp clients. [...] As of [Apr 2016], 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. This includes all the benefits of the Signal Protocol – a modern, open source, forward secure, strong encryption protocol for asynchronous messaging systems, designed to make end-to-end encrypted messaging as seamless as possible."