"You have to backport security fixes for your own tiny platform because your build environment doesn't support our codebase or make your build environment support our codebase" seems like a 100% reasonable stance to me
If that is due to the build environment deviating from the standard, then I agree with you. However, when its due to the codebase deviating from the standard, then why blame the build environment developers for expecting codebases to adhere to standards. That's the whole point of standards.
According to git docs they seem to try to make a best effort to stick to POSIX but without any strong guarantees, which this change seems to be entirely in line with: https://github.com/git/git/blob/master/Documentation/CodingG...
If your codebase used to conform to a standard and the build environment relies on that standard, and now the your codebase doesn't anymore, then its not the build environment that deviates from the standard, its the codebase that brakes it.
Because they don’t. For instance, if they were fully POSIX compliant, they’d probably already have LLVM.