Could assume that.

S&box was initially developed on top of Unreal Engine, but in a backend-agnostic design. It's more like a framework/runtime meant to be portable to any backend engine. Once Source 2 released, S&box was ported to that.

I wouldn't call it an engine because of that. S&box is open source, but you can't run it without a closed-source backend.

Valve isn't keen on releasing Source 2 as widely as Source, and I feel like soon, S&box will be declared the official API interface for the engine, while the backend remains unstable. Kinda like Win32 vs NT.