If anything, the Mono runtime is what prevents heavier (ab)use of SIMD types in the standard library, or at least it causes additional required effort to make it not regress in performance since it's nowhere near as capable as CoreCLR.
As C# SIMD had a such a long head start in Mono (your linked MS blog post has their timeling starting only in 2014) it should probably get a bit more credit.
It was after all enabling games to use SIMD with C# (with MonoGame and Unity), even if their interface wasn't the winning one in the end.
You can find the overview of relevant high-level types here: https://learn.microsoft.com/en-us/dotnet/standard/simd
For low-level types and historical context: https://devblogs.microsoft.com/dotnet/dotnet-8-hardware-intr...