Your same argument can be applied to virtually any other language:
This ruby benchmark doesn't matter because ruby is written in C!
This nodejs benchmark doesn't matter because v8 is written in C/C++!
etc.
This ruby benchmark doesn't matter because ruby is written in C!
This nodejs benchmark doesn't matter because v8 is written in C/C++!
etc.
No it can't. Each of those languages you listed (as well as Python) has its own runtime system designed to implement the desired semantics and features of the language. When writing a C extension to one of these runtimes, you're escaping the confines of the runtime system in order to improve performance or capabilities not native to the host runtime.
The point isn't "because it's written in C"; the point is that it's not written in Python and therefore the results of the benchmark cannot be attributed to Python, to say nothing of the merit of the benchmark in the first place.