That is a reason one might make a racist joke, but certainly is not the only. There are countless reasons. Superiority theory doesn't explain all racist humour, only a subset.

There is no unifying theory of humour. There's absurdity, use as a coping mechanism, its use as a measure of social coercion, and countless other reasons. Asserting superiority is a possible motivation but far from the only.

I'm not saying the show itself is racist, just that the joke was parodying how certain racist idioms embed themselves within culture and become so normalized that one doesn't even think about using them until confronted by a member of the group caricatured. That's the joke. She thoughtlessly uses a racist idiom (in this case that elephants find offensive) without realizing an elephant is in the room. He gets seriously offended and leaves. She doesn't intend on causing offense but doesn't think about the connotations of the words she uses.

That is a clear parody of racist idioms, jokes, and sayings that people sometimes thoughtlessly say without intending on offending people.