The debugger is proprietary but still works cross-platform. I don't know how Jetbrains does C# debugging in Rider exactly, but that shows that you don't have to use VS (Code) to do C# development if you don't want to.

Thanks to Samsung of all companies, there's an open source C# debugger on GitHub (https://github.com/Samsung/netcoredbg). That seems to be the basis of the open source C# extension's debugging capabilities: https://open-vsx.org/extension/muhammad-sammy/csharp

The VSCodium C# community wants Microsoft to open source their debugger instead of having to maintain an open source version themselves, but that doesn't mean you need to use Microsoft's open source version. If anything, this forceful separation makes it so that there never will be only one implementation (like there is for languages like Rust which have always been open and therefore only have one way of doing things).