My personal rule is that the sound level should be comfortable without ANC. So, if the sound is physically dampened sufficiently by the earbuds or headphones even when ANC is disabled, I'm happy.
ANC is more for comfort than safety from what I understand.
I can't say that the feedback causes damaging levels of noise, and its possible that my hearing is still protected, but the feedback that is produced is so uncomfortable that I involuntarily pulled them out of my ears.. which is not a great reaction with power tools in hand.
Apple does market 'Hearing Protection' as a feature of the AirPods Pro. https://support.apple.com/en-us/120850
Looking at the ANSI Datasheet, it seems like it's not really suited to the sounds coming from all power tools since they can reach or exceed 110 dBA:
> The Hearing Protection feature is not suitable for protection against extremely loud impulse sounds, such as gunfire, fireworks, or jackhammers, or against sustained sounds louder than 110 dBA.
I suppose if you know your tools operate at 110 dBA or lower levels, you're good to go. The feedback is bizarre though... Hopefully Apple resolves that with a firmware update!
Edit: It looks like it can safely attenuate sound at 110 dBA for 16 to 63 hours. That's impressive!