I imagine that that's probably true but I didn't really notice a large difference.
The thing I like about tracing paper is that it doesn't matter if the toner isn't that thick and you can it's easier to shade in the missing parts with a fine pencil, than it is to colour a transparency with a permanent marker.
I've used tracing paper and vellum and have fouind the toner just not dense enough to provide the contrast for good, consistent results. I've also used vellum as a "release" agent for the "toner as the resist" method and have even used tracing paper for that method with no mechanical lifting of the paper (disolving the paper away with suphuric acid). But, do note that not all brands of tracing paper are the same. Some are akin to vellum and others more like very thin paper.
But, even with the paper, it surely wouldn't hurt to beef up the density of the layout by overlaying the foil.