Here's Michael Crowley's response which I thought is interesting.

https://newrepublic.com/article/62416/cock-and-bull

Ugh, that's clearly a joke that got converted into a factoid as it passed through a dim NYT reporter and then Wikipedia

This is a bad flaw in Wikipedia -- it only cares about sources, not credibility of sources.

Why can't the small penis be part of the libel?

It can, it's just that the plaintiff has to prove they have a small penis.

If the small penis is part of the libel then surely the plaintiff has to be prove he doesn't have a small penis.

Which doesn't actually make any sense. It's a fun joke, but you could make the same "it's just an identifying feature" argument about any libelous part.