The current xpath just looks for the value 81:
LET audiobook be the result of calling check for node on onix, /ONIXMessage/Product/DescriptiveDetail[PrimaryContentType = "81" or ProductContentType = "81"]
But 01/81 indicates an audiobook while 09/81 is the code for wcag 2.1, so this triggers false positives in non-audiobooks. ProductFormFeatureType = "01" should also be specified.
See #670 (comment) for the possible issue here.