This is a bit afield from the amendment process discussion, though I think it's highly valuable to improve and apply clarity in amendment cases.
But I don't think the case cited is pedantically speaking an amendment to RFC-0111 by RFC-0151. That is, RFC-0111 is on the general subject of how arch-specific requirements should be developed, and it has as examples some details about ARM and x86. RFC-0151 doesn't change that general rationale, and as examples what RFC-0111 says about ARM and x86 are still valid examples. RFC-0151 actually formally defines the individual arch-specific requirements, IMHO for the first time actually formally, for ARM, as RFC-0073 did for x86.
That being said, amending the text of RFC-0111 both to make its examples consistent with current actual arch-specific details, and to be more clear that it is talking about the general case, seems helpful.
I'm not convinced we want to change RFC-0111's policy to say that cryptographic features are strictly required in any future arch-specific requirements defined for a future architecture, though perhaps we do and that can be debated. But I definitely don't think we've already changed the policy RFC-0111 formalizes in that way. It's just that RFC-0073 and RFC-0151 define arch-specific policies that are more stringent as instances than the RFC-0111 requires of the whole class. So IMHO it does require a fresh review process (AFAICT another RFC) to agree to change the RFC-0111 policy.
Note that the recent RFC-0211 defined the initial arch-specific policy for RISC-V. Though that is very likely to be amended later as the RISC-V support develops, it as ratified by RFC does not require any cryptographic ISA features. So if RFC-0111 were amended today to indicate general policy change, RFC-0211 and RFC-0111 would be in conflict. If RFC-0111 is simply updated to make its examples match current policies and/or to clarify that it is not the authority on arch-specific policies but only the policy guidelines for how arch-specific policies should be decided, then there remains no conflict.