I wouldn't really agree on this point.
Unsafe blocks have been a feature since C# 1.0 They may not be the default way of writing C#, but they've always been a natural and integral part of C# and have extensive use internally to ensure that interop can work and extra performance is available where possible.
SIMD acceleration has likewise been a part of the BCL (standard library) for nearly 10 years now and is just as integrated behind the scenes. It being part of the formal BCL makes it even more "standard" than the equivalent in C/C++ where such functionality is relegated to compiler specific headers/extensions.
SIMD code itself is also very idiomatic C#. There is nothing really different about utilizing the APIs, the only difference is in how you think about handling your data. Needing to think differently about how data is handled for some contexts is applicable to many domains in C# (and programming in general). It's no different than making code work with async or multi-threading ;)
Simply put, all these features are still C# and I don't think its necessarily good to say that using them means you're not really writing C# anymore. I view it as a disservice to the language, its extensibility, and power. It also leaves a connotation that you might be better off writing C/C++ instead, which is often not the case.