The glibc core maintainers have such responsibility, yes, but it's too much burden for occasional contributors focusing on a single issue or a particular architecture. Setting the bar too high would make glibc lose valuable patches from both Intel and AMD.

A reasonable compromise is requiring architecture-specific contributions to at least do no harm to other vendors, and to not increase maintenance costs for core developers by duplicating code.

In this light, AMD engineers wanting to enable the haswell optimizations for their processors would be asked to share the existing code rather than copy-paste it. Intel engineers would participate in the public review to ensure AMD patches don't cause regressions for Haswell. If they have contributed testcases, they will demand that AMD patches pass them on all supported architectures before being merged.

This is pretty standard in all open source projects with multiple stakeholders.