Most commercial engines that use libraries like havok will wrap them in some sort of internal library like this one to shield themselves from types from the middleware leaking into engine code, implementing convenience wrappers, and making it easier to update/swap them at some point if they ever wish to.

There is a third-party implementation of vphysics based on an open source physics engine, for example - so for Valve that seems to have been successful: https://github.com/misyltoad/VPhysics-Jolt