If the thread was about a dead horse and someone said eat it, to which OP replied "I'm not British", no one would bat an eyelid... Mention another nationality and suddenly it's "inappropriate".
The difference being:
1. The horse meat scandal was widely publicised and dominated the news for weeks, making it a decent and obvious punch line, unlike a single vague article from 4 years ago
2. 'Hilarious' English people are always joking about cats/dogs going missing near Chinese restaurants, it's crude stereotyping.