Think about it this way.
It's fine to eat the chicken cold. It has been cooked and there are no nasty bacteria left. (Raw chicken is risky because the bacteria in it need to be killed by cooking. But cold cooked chicken is fine up until the sell-by date.)
It's also fine to eat the chicken hot.
So suppose DH had put it in the air fryer for one second. It would still be fine, right?
Two seconds? Fine. Yes? Two seconds of heating it can't hurt.
Five seconds? Fine. Yes?
And so on.
For there to be a problem, there has to be some temperature between 4°C and 75°C at which, suddenly, the chicken becomes dangerous. Which there isn't. The chicken doesn't know that it's been in the air fryer for "less time than it should".
If the food is safe when it's cold, it's safe when it's hot, and it's safe when it's warm. There is no point in between at which it suddenly and immediately becomes dangerous. If chicken went bad that quickly, you'd have to throw away a meal that got a bit tepid because you had to answer the phone. In fact there would be huge disclaimers on it: "Once heated, eat within 3 minutes".
(Obviously you don't heat it up and then leave it on the windowsill for a week.)
The confusion arises because the food manufacturers have standard texts that the health authorities (not unreasonably) make them put on everything. It's a bit like having to put "Warning: Contains peanuts" on a packet of peanuts.
Something similar happens with frozen food. "Never re-freeze food once it's defrosted" is generally solid advice. But if you buy chicken breast fillet on the 5th of the month that has a use-by of the 18th, then freeze it on the 6th, defrost it on the 7th, change your mind and freeze it again on the 8th, then defrost and cook it on the 9th, it isn't going to turn into plutonium.