Realistically the best thing you can do is let it go and have the satisfaction you were right. No interview is perfect, there are false positive and negative hires. Don’t take it personally.
Name and shame them, OP.
But would be a total psycho with an exhausting life for most people... So best to just learn that ideas are a dime a dozen, dont give them out for free if they are good
I don't think they should be obliged to hire you just because you had this one idea that helped them.
It's possible they were not interviewing you in good faith, but just wanted free advice. But that's impossible to prove, and several other explanations that don't require bad faith are entirely plausible.