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.