This is key: "on all of our remarketing campaigns"

The discrepancy is that a 75% reduction in users opting into Facebook tracking is leading to only a 30% reduction in iOS share on the highest ROI ad campaigns. 30% is big but not 75% big.

Another possible explanation is that opting out of tracking in iOS doesn't make you invisible. It just makes it hard for Facebook to correlate your off-app/non-Facebook.com web activity, but there are ways around that. So what 30% means hinges a lot on how the 1st touch -> 2nd touch leap is done, and the data for that leap doesn't always come from Facebook.