It's not nessessarily the best benchmark, it's a popular one, probably because it's funny.

Yes it's like the wine glass thing.

Also it's kind of got depth. Does it draw the pelican and the bicycle? Can the penguin reach the peddles? How?

I can imagine a really good AI finding a funny or creative or realistic way for the penguin to reach the peddles.

An slightly worse AI will do an OK job, maybe just making the bike small or the legs too long.

An OK AI will draw a penguin on top of a bicycle and just call it a day.

It's not as binary as the wine glass example.