It's more about right tool for the right job.
Good example is Azure CLI; it's Python. Microsoft is also a big contributor in the Python scene[0]
I don't think it's surprising at all that they didn't use C# to write a compiler for TS.
They have internal champions for Rust[1]
I'd say Microsoft is possibly one of the most diverse shops when it comes to tech selection.
[0] https://devblogs.microsoft.com/python/supporting-the-python-...
[1] https://www.theregister.com/2022/09/20/rust_microsoft_c/