It's a larger vehicle for the transmission of the genre, while at the same time not being a genre band. I've talked to some avid black metal fans that are put off by this aspect of the band -- that it is a band that doesn't fit every traditional idea of black metal still co-opts it.
However, in an age of terrible genre-throwback bands that seek legitimacy by authentically replicating some bygone era in music, I welcome Deafheaven's choice to not completely rip off a band like Darkthrone.
Singer George Clark's Ian-Curtis-turned-serial-killer appearance and jerky and vaguely sexual mannerisms while performing are endearing and the parts of the songs that involve shoegaze, post-rock, or hardcore elements are just as engaging as the overtly black metal parts.
It's music that is aware of its context and wears its musical heritage proudly while still trying to be forward thinking. Just as Ezra Koenig of Vampire Weekend does not pretend to have come up with the term kwassa-kwassa, Deafheaven guitarist Kerry McCoy isn't laying any claims on tremolo picking.
In both cases, the genres are sources of inspiration, different ways to think about crafting a sound. Seeing Deafheaven made me realize that black metal can be, above all else, very pretty and evocative music, and trying to implement that prettiness in a project that is outside of that scene can be wonderful when done right.