There's only double bass drumming in stuff like death metal, deathcore, and stuff like that.
And metal doesn't have to be more technical than rock at all...
Well I'm talking generally about metal. By rock, people usually refer to bands like Beatles, Floyd, U2, etc. And by METAL people usually think of bands like the Big Four, Sabbath, Iron Maiden, etc. Now compare those bands and see which ones are more technical. They don't HAVE to be more technical, like Agalloch (post-metal judging from The Mantle), but they are just more technical because of their nature to be the fastest and the loudest kind of music.
Double bass drumming doesn't have to be all-out 32nd notes blast, it could mean just incorporating the technique in some parts of the songs. Like in One.