By a certain definition, they are. They obviously can't run springboard or UIKit, but they share the same underlying technology; that is how they can run Siri and have a very tight integration with your phone.
They're certainly tiny computers, but has it ever been established (or even seriously speculated?) that they're running xnu? That's the minimum hurdle they'd have to clear for me to feel at all comfortable with the statement that they "run iOS".
I strongly doubt that they were able to shrink Darwin/iOS down to the Cortex M0+ that the AirPods have. I would love to be proven wrong, though.
Tiny Darwin computers, maybe. Gruber knows this but he's writing for a broader audience.