As of 2020 (when I stopped using F# and .Net in general), the CLR only had inheritance and F#'s enums were actually implemented using inheritance. Eg Some and None were derived classes of Option class. You would see it if you inspected an F# assembly in a decompiler. I don't know if it's still the same today. This C# proposal has anonymous enums using `A or B` syntax which would be hard to make work as sugar over inheritance, so I guess the CLR would support enums first-class for this to work (if it doesn't already).
This is only partially true. Both Some and None were always instances of single FSharpOption class (None is actually not an instance, but null), however custom DUs are indeed compiled into classes with inheritance
