Sammy
yes, to be fully compliant with AC91-57 you must obey part 3b about airworthiness.
Unfortunately, nobody is actually required to obey AC91-57.... there is no teeth behind the AC to mandate compliance.
If you want to look for the teeth, read up on AFS400 / Guidence0801 and how they use AC91-57.

To consider AC91-57 to apply in a 'must abide' way
one would first have to look at the 400' alt limit everywhere in the AC,
and how AMA is simply choosing to not abide it.