Maybe they meant the term more literally, like along the lines of "the subject material on this album is far weightier than that of Lateralus"

Let's face it, Lateralus was pretty much all airy-fairy stuff about transcendance and spirituality. Not that that's a bad thing by far; Lateralus is probably my favorite song ever, but the subject material on 10,000 Days is largely Media, war, death and fear...