The first official implementation of directly connecting WhatsApp to another chat system - using APIs built specifically for purpose instead of third-party bridges - was indeed done against the Matrix protocol, as part of a collaboration in testing ways to satisfy the interoperability requirements of the EU Digital Services Act.
So not a case of a third-party bridge trying to act as a WhatsApp client enough to funnel communication, but instead using an official WhatsApp endpoint developed - by them - explicitly for interoperation with another chat system.
I think the latest update on the topic is the FOSDEM talk that Matthew held this February.
Edit: It’s worth noting that the goal here is to even support direct E2EE communication between users of WhatsApp and Matrix, something that’s not likely to happen with the first consumer-available release.