> Steam recognizes the Source libraries as a distinct element
I don't think that's correct, all Source1 games include their own copy of the engine and aren't automatically updated to newer versions etc. (Valve does update them though). The engine download is only for mods and some 3rd-party games.
The games you mentioned _used_ to run on the same branch of the engine, and when that does happen you only download it once. Most shipping Source games today use distinct branches of Source. There's a Portal 2 branch, a DotA branch, a TF2 branch, a CS:GO branch, etc.
I don't think that's correct, all Source1 games include their own copy of the engine and aren't automatically updated to newer versions etc. (Valve does update them though). The engine download is only for mods and some 3rd-party games.