He's saying complete coverage with deadner will turn it into more of a drum surface.
I don't agree completely, as I've found the 50 to 75% coverage works with older vehicles nicely without adding a huge amount of weight. The chief thing is any large open panel area, outer door skins for example make the biggest improvement.
Going between ridges and bracing, slightly covering the brace, will also help without adding thickness to the floor covering.
On our trucks, I think the best method for the floor is urethane bed liner and use the adhesive deadner on the firewall, doors, and side panel.
They make some light weight stuff that works well on the roof. Also the foil bubble insulator attached to the roof with 3M contact cement works very well on the roof and provides pretty decent heat rejection.