It is not just WINE. The Steam Linux Runtime is a stack of linux native libraries, binaries and tools designed to give game devs a consistent version of things to develop games against. Recently they moved this to be container based and I believe proton (which contains wine) is run inside this runtime as well.
This is irrelevant with Steam though. Steam offers a runtime with preconfigured versions of everything that is needed to give the devs a consistent environment for their games to run no matter how fragmented the linux install base might be. This runtime is also what proton uses for ship its different versions.