1) It's possible (however unlikely?) you weren't the only candidate to mention the idea
2) I sometimes ask candidates how they would handle tricky dilemmas I face; yeah, it's free consulting in a way but it really helps you see how the candidate thinks. If the candidate can't tell you something you don't know, why are you hiring them? But the ability to answer that question well may not be the only skill or the most important one for the job; maybe they were indexing more on another skillset than that idea itself and its execution for the role they were hiring. This is just part of interviewing candidly. I think you are better off showing your expertise than hiding it.
3) Hindsight is 20-20; if you hadn't mentioned the idea, you still wouldn't have gotten the job. You took a risk to tell them and it didn't pay off, but you didn't get harmed. I agree it does seem unfair/unjust, but this is a little bit of the nature of ideas. If you want an acknowledgement/thank-you, I would circle back to the CTO and say "hey, I saw you ended up hiring someone else, but you did seem to like one of the ideas I mentioned (xyz) since it looks like you all are pursuing that... can you give me any feedback on what I could have done better in the interview?" That will give him a face-saving way to acknowledge your perception or refute it without you accusing him/her outright without direct evidence or threatening him... You might still not get an answer... but you might get a better sense of what happened, from the ensuing admission or the ensuing stonewalling/denial or failure to answer.