It does and counted for me. You have to make sure you are killing enemies inside the zone and the zone is red at time of killing. Once neutral or any shade of blue move along there is nothing to kill here.
I am quite certain you might just be on a BLUE point instead of a RED one, because remember that even though you haven’t fully captured a point yet you are still Defending it if the point is blue
That’s going to be a nightmare to complete, then. Driving a tank right onto a red-hot CP is a surefire recipe to die to a dozen Assaults unless you have 1-2+ pocket engineers.
Tanks counted just fine for me, though given how much air I was catching it would make sense why they wouldn’t.
Make sure the objective is held by -or being captured by- the enemy or they won’t count, once you -or your team- starts capturing the objective the kills stop counting.