It is true, plus it needs to be applied to BARE metal to be effective. Applying over coated surfaces is a waste.Apparently, POR15 doesn't offer UV protection:
"WILL THE SUN DESTROY MY POR-15 COATING IF I DON'T TOPCOAT IT?
If the surface is exposed to the sun for an extended amount of time, the UV rays will eventually break down the POR-15 and cause it to fail. That?s why we recommend you topcoat it."