He's out of his tree. There are now a couple of different companies
that make replacement blades for the more popular Bell helicopters -
completely PMA'd parts. But you are still going to need a heck of an
STC in order to mount one on your Cessna 172. G

If I just had to guess, I would say that where he got confused is that
if you are replacing a part on your aircraft with a part that is a
PMA'd *replacement*, then you don't need an STC. But that's because
it's already covered by the TC (or by a previous STC). The PMA says,
in effect, you can treat these parts as APPROVED parts (including
approved replacement parts). Doesn't necessarily mean you can stick it
on your plane anywhere you feel like.