MSBuild assumes you want a 1-to-1 relation between projects and assemblies, so assemblies and projects are interchangeable. There is no hybrid C#/F# compiler than can produce one assembly out of source code from both languages, so you’ll always need at least 2 compilers to be involved, hence the need to have a csproj and an fsproj side by side in your code base.

Regardless, major kudos for your rewrite!