Is this based on your experience having tried both? Because that reasoning is questionable. IEMs don't block all sound from hitting your eardrum, some of that sound will hit your eardrum via bone conduction. And ANC headphones do block some sound passively, the inner ear ANCs can often create an airtight seal.

Yes, I have tried both. However the only ANC I have experience with is AirPods Pro, so that's all I have to go on. But I can definitely say that IEMs do a better job than the AirPods.

Which IEM and tip did you get? Do you know why the IEM was better given that the Airpods Pro also plugs the canal?

Shure SE215 and the default tip it comes with. I can only speculate on why the IEM is better, but I suspect it comes down to the tip material creating a better, more substantial seal. The outer shell is also physically larger, and that may also play a role in keeping more sound out.

I wonder if we can replace the tip on the Airpods Pro to create and IEM like seal. I suppose that should be the best of both worlds? But the outer shell would still be smaller so maybe not.