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".
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.