Airpods can't compete with a decently priced boom microphone that actually comes close to your mouth. The distance from your mouth to your ear (where the mic resides) is quite long especially considering how little space they have to throw in a capsule into.
So, either you'll get a lot of ambient noise, the signal is quiet or Apple will do some algorithmic trickery that tries to approximate some kind of echo cancelation on the audio signal, but compared to a simple dynamic microphone that just has a more favorable position and form factor, it'll always lose.
Hmm. I just measured the distance between my Airpods and the corner of my mouth at 3" or around 7.5cm. Very approximate measurement, but it seems to be not far off of recommendations for where to place headset boom mics (google says 1-3 inches from the mouth.)
Also, it's worth noting what the goal is here. The aim is not to capture the most accurate sound period. I've had calls with people who clearly have very expensive setups, but I end up hearing pen clicks, keyboard sounds, breathing, swallowing etc. The Airpods seem to do a great job of making my voice sound good in general. I've gotten compliments on my audio (so it can't be that bad) and the Airpods don't seem to pick up my breathing, typing, etc so I'm happy.
So, either you'll get a lot of ambient noise, the signal is quiet or Apple will do some algorithmic trickery that tries to approximate some kind of echo cancelation on the audio signal, but compared to a simple dynamic microphone that just has a more favorable position and form factor, it'll always lose.