This rule is incomplete. Most two-syllable adjectives ending in "y" can be converted to comparative form with "er". Some of these may be uncommon, but not all, and my spell checker agrees they are real words, in both British and American English.

Eg. Angrier, heavier, cleverer, friendlier, happier, lazier, tidier, etc. And even three syllable words can take "er": bubblier, foolhardier, jitterier, slipperier, many words starting with "un".