If a job you apply for a job and it turns out it's not what it's advertised to be, there's nothing unethical in declining the job. The fact that the platform doesn't have a way of saying "nevermind thanks, not what I signed up for" is not the authors fault.

They were explicitly looking to do work for an AI, when it turned out to be a human driven marketing stunt they declined.

They didn't decline because the idea "came from a brainstorm" with a human, that message was much later.

They declined because the note on the flowers had a from line that was an AI startup. When you were otherwise on board with an unsolicited flower delivery and a social media post to make the sender look good, that's a picky reason to deny it, and saying it's "not what they signed up for" is a pretty big exaggeration.

Except they didn't decline, they ghosted, and that's just bad behavior.