> copywritten

This syntactic mistake is driving me nuts in the article is driving me nuts. It's a fundamental misunderstanding of what the the word means.

It's "copyright". It's about the rights to copy. The past participle is "copyrighted", someone has already claimed the rights to copy it, so you can't also have those rights. The rights belong to someone else. If you remember that copyright is about rights that are exclusive to someone and that you don't have them, then you wouldn't make such a syntax error as "copywritten".

The homophone "copywriting" is about take a sales copy (that is, an ad) and writing it. It's about creating an advertisement. Copywritten means you've gone around to creating an ad. As an eggcorn (a fake etymology created after the fact in order to explain the misspelling or syntax error), I assume it's understood as copying anything, and then writing it down, a sort of eggcornical synonym of "copy-paste".