Genera definitely had a C (and I think C++) interface.

Written on top of Lisp.

A lisp machine, by definition, runs lisp. The ABI problem only exists in environments where multiple programming languages exist.

I doubt much non-.NET software runs on that C# OS either.

If you want multiple programming languages to be able to sanely run within a single OS (and do more than just pure computation), though, you have the ABI problem, and this is what the whole discussion is about.

.NET was created as a Common Language Runtime.

When it was announced it supported about 27 languages.

From Microsoft themselves, J#, C#, VB.NET, and Managed C++ (later replaced by C++/CLI).

It was WebAssembly before its time (among many others), with tons of features that WebAssembly is yet to support.

The Symbolics lisp machines had C and Fortran compilers

Implemented in Lisp.