DSBM makes up a significant chunk of black metal. Atmospheric black metal is no riffier. Those two alone probably account for at least a third of all full black metal (e.g. no black/thrash obviously), if not closer to a half. Symphonic black metal is a little riffier, but far from consistent. Let's imagine that 90% of sympho and folk black is worse riff-wise than that Liturgy song. Now we're looking at around two-thirds of all black metal accounted for, and about 95% of it is automatically inferior riff-wise. What remains? You have canonical second-wave clones; certainly some would beat Liturgy, but many would not, trudging endlessly through a million minor Transylvanian Hunger copycats. I'll be generous and say that only two-thirds of those bands are worse than Liturgy, and guesstimate that they make up approximately a quarter of all black metal. Of the remaining sub-10% of black metal exist a minority of blackgaze (nearly riffless), "Hellenic black metal" (aka how to repeat two Manowar/Bathory riffs forever), tech-black (riffy but rare), and probably some other shit I'm forgetting. Add it all up and my 90% figure was probably slightly hyperbolic, but it comes out to about 85% which is still close.