Most likely, 80% sure, they count as Dtilt.
I think that's not accurate. Bullet arts stale the move you are using (even if one bullet touch it counts like one move)
Example:
-after 10 bullet arts Nair => Nair does 4%
-after 10 bullet arts Nair => Dtilt does 6%
-after 10 bullet arts Dtilt => Dtilt does 3%
-after 10 Dtilt => bullet arts Dtilt does 5% (instead of 10% not stale)
Also, if you hit the move AND the bullet art of that same move, it counts as 2 moves in the stale list it seems but it weird, sometimes the numbers don't correspond:
-after 4 Dtilt => Dtilt does 4%
-after 2 Dtilt with bullet art => Dtilt does 5%
-after 5 Dtilt => Dtilt does 4%
-after 3 Dtilt => Dtilt does 5%
-after 2 Dtilt with bullet art + 1 Dtilt => Dtilt does 4%
So yeah, choosing wisely the moves you bullet art at kill percent can make a difference.
PS: correct me if I'm bull****ting somewhere