As you note, they didn't conform to a BCP. There is no requirement (or
implication even) that one should conform to a Best Common Practice.
These serve to distribute helpful advice to the community and document
practices that have worked for others, in situations that may be
different.
The AXFR issue is compliance with a required standard, and its definition.
Much more serious concerns and issues are at stake.
Note: The IETF has never made *any* standard that the IETF required people
to conform to, unless they wanted to truthfully claim conformance to that
standard.