>I'm guessing that for Korean it's mostly the grammar differences, honorifics

Definitely! And some other differences too. As someone living in South Korea currently, the reading/writing part is trivial, but beyond that it is a lot harder to learn (for an english speaker (although I'm danish)) than, say, French.

I definitely disagree that Korean and Spanish would be considered equally hard, and know literally no one here that would say that either.

Maybe that person is the speaker of Arabic. For them, Korean and Spanish should be equally distant.

The writing system is extremely easy compared to Chinese and Japanese. It's just an alphabet.

Korean writing just looks odd, like funny Chinese characters. But it's just an alphabet with letters grouped into syllables.