Contributed 3rd Party Builds • Re: Pale Moon x86-64 SSE2/AVX2

And if that's the case, couldn't the building script(s) detect that option and if necessary correct

Theoretically, yes, but then you're asking the build system to be aware of and respond to every compiler option a user might be passing in the optimization flags. For every compiler supported, on every OS supported. That would be neither simple nor very maintainable. Asking the person building to pass the correct/logical configuration parameters in their mozconfig is simply the most common sense solution.