Understanding either concept takes the same level of intelligence if you understand the meaning of the words (both a vegan wolf and a meat-eating goat are nonexistent entities outside of possibly bizarre exceptions, yet someone capable of understanding will have no problem with either).
That GPT has no trouble with meat-eating goat but struggles with vegan wolf hints that the former has some "statistical" property that helps GPT, and which the latter doesn't. It also hints that GPT doesn't understand either term.
Hence my example: something a human wouldn't fail to understand but GPT does.