Earlier quoted context omitted.
> They are complying, Are they, if they removed the Zig copyright from each file in the standard library?
Yes, because, as I explained above, they still retained the original Zig copyright in their LICENSE file. The MIT/Expat license doesn't say anything about the file headers. It only requires that "The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software."
That the individually-protected work is also included in an compilation which is itself licensed under MIT does not remove or loosen any licensing requirements on the smaller work.