Is there anything I can do?
Letting it go is the simplest thing that might work.
There are a lot of alternatives of course. Many of them will suggest not hiring you dodged a bullet.
Also there is a difference between not acknowledging they implemented your idea and not remembering they did. Acknowledgement would have involved a somewhat awkward conversation seeing as you weren’t hired…
…this time. Maybe they reach out in the future. Maybe you run into the CTO in another context…and it would perhaps be productive to add the CTO to your professional network. The future contains everything positive that might happen and the what’s past is done.
It sucks not to get a job. Good luck.