There’s a difference between source available and open source. For example, actually being allowed to distribute modified versions is pretty damn important:

Restrictions

• No Distribution of Modified Versions: You may not distribute modified versions of the software, whether in source or binary form.
• No Forking: You may not create, maintain, or distribute a forked version of the software.
• Official Distribution: Only the maintainers of the official repository are allowed to distribute the software and its modifications.

• C# = Java (the language itself)
• .NET (Core) = Oracle JDK (a runtime and std lib implementation for the language)
• ASP.NET = Spring boot (the default web framework)

Rust: works

Zig: segmentation fault