As far as I see, Naudet is telling us that HITTING more than 10 times is needed to cause damage, not that his aiming is good or bad. And I agree with him.
In the other hand, IMO, only 2 20mm hits on the tail is more than enough to destroy almost any plane elevator (at least half), 4 solid 20mm hits on a 190 tail will destroy it for sure and the rudder too.