They're also (?) using Euphoria: https://en.wikipedia.org/wiki/Euphoria_(software)

I remember this being made a semi-big deal when IV was coming out.

Are they using both, for different parts of physics simulation?

Every game released using unity or unreal uses PhysX rather than Havok. Physx has a sizeable market share in AAA games too.

PhysX is disliked due to it being partial towards Nvidia-based h/w. Also the console SDK (of PhysX) is very compute-intensive and usually not the best solution.