"90% of medieval English altars contained hidden stone phalluses" factoid actually just statistical error. Most medieval English altars were destroyed in 1552. Phalloi Georg, who lives in a cave and has an altar with over 100,000 stone phalluses, is an outlier and should not have been counted.
Every so often I encounter an anecdotal claim that is repeated again and again across secondary texts in a maddening loop of apparent reinforcement, and yet an original source reference is always somehow absent. Here's an example, involving altars and stone phalluses (thread)