To be fair to him, what he meant was that if you had a product that was
PMA/STCed for a given airframe (for example the B&C standby alternator
referened at [1] below) then the installation of this product did not
require a 337, even thought it was being installed using an STC. He
says that the PMA approval removes the need for additional paperwork.
My view is that you still need the 337 if it's a major alteration,
which this would be, and that the PMA-ness of the parts is merely
something that allows the manufacturer to put them on the market,

[1] http://www.bandcspecialty.com/PA32desc.html