Just stick to the truth. You retired early, but this job sounds so interesting, you're going for it. Be honest. I'm sure they'll find that refreshing. It will also make you memorable, which is most helpful. For your last entry on your resume, list the start date and the retirement (not end) date. Easy-peasy.
When you get to the interview, you can have some fun with this. "The fact that I'm applying for this job post-retirement proves my motivaction is strong. How do you know those other candidates aren't just applying because they need the money?"