He was a good guy, wrote good code, was fun to work with. But at the same time, his FIRE status was front and center every time it came to task assignment and other issues. His manager was always afraid to give him un-fun tasks or push back when he was behind schedule for fear he'd quit. Not a great dynamic for the rest of the team who didn't have as much leverage and therefore received a disproportionate amount of the un-fun work.
If a candidate came to me suggesting that they didn't need this job but they only wanted to work if it was fun, I'd honestly have to proceed very cautiously. The last thing I want to do is hire a candidate who opens with an indirect threat that they're going to leave the second the job isn't fun any more. We have one of the better team environments, but I can't guarantee that work will always be fun and never require some hard weeks or months to get things done.
I'd personally not mention it. It's not only irrelevant to the hiring discussion, but it's a weird warning shot to your future manager before you're even hired.
I always thought that was kind of interesting, that being overqualified for a Walmart job might make it harder to just get a Walmart job. (I briefly considered it in a pinch but decided to drive for Uber instead.)
I think maybe his manager misplayed and could have approached this from a different perspective. "Ah, I have someone who I can keep at the same pay who is willing to work for the hell of it." Or something like that. Definitely feels like that's the kind of thing you could negotiate with an employee who's joining just for the work and not for the money.
I hit my FIRE number about a year and a half before leaving my last corporate gig, and have a BATNA (Best Alternative To Negotiated Agreement) of, basically, "fuck you, fire me" meant I went into my last couple of salary/role/location negotations with enough excess confidence that I was always able to press for what I wanted (within reason), presumably because everyone opposite me could tell they didn't have a lot of wiggle room.
It only works until it doesn't, though -- I left that job when they closed my office out from underneath me and straight-up relocated my role elsewhere. They asked me to move with it, and I said "no thanks". Obviously they called my bluff (I was being totally unreasonable, to be clear), and...now I'm self-employed.
On the upside, I did mention that I'd already hit that FIRE number, so...