Yes, you're correct - only DeathProof really qualifies as "grindhouse" but the correct description - "movies featuring copious quantities of gratuitous violence" - is a little wordy....
Well, I am a genre nazi, and no its not, grindhouse refers to a specific era when certain cinemas played cheap films and the film makers "grinded" them out Deathproof is a tribute to the era, but I'll wager it never played at a grindhouse theatre, if any exist anymore.
They weren't even called grindhouse at the time, its a new catch phrase.
Its a mainstream movie, that pays homage to a long gone era of low budget shockers,
If a movie plays at your local multiplex, it AINT grindhouse.
(sorry as i said I am a genre Nazi)