Presumably it depends on the modulus used? I think the article is assuming a modulus of 2^n-1. As the article says, Adler-32 is a specialization of Fletcher-3

Presumably it depends on the modulus used? I think the article is assuming a modulus of 2^n-1. As the article says, Adler-32 is a specialization of Fletcher-32 that specifies a modulus of 65521 - that would seem to avoid this limitation.