Welcome to PF from Omaha, NE.
I, too, think that Traeger is warning you that foil on the drip pan can fill the gaps around it and block proper air flow. Members here have written about this happening, not necessarily on Traegers.
I quit covering my drip tray with foil because grease got under it and it was the same clean up.
A rack on a drip tray sitting on the grates, per Canadian John, seems like a good solution. I trust everything he writes.