Er... yeah. That was news twenty years ago.
But not quite stated right. Methane clathrates form within a fairly narrow window of pressure and temperature. So only along continental margins. And since P&T vary over time, they're a relatively short-term reservoir, in geological time.
As far as carbon sinks go, formation of carbonates on the margins, driven by weathering of continental rocks (and subsequent export to the ocean, through rivers) is much more significant. But not as exciting as rocks that burn.