how do you test that it doesn’t work

The mission includes some comments with testing instructions. I guess I'll write them out here as well....?

The vehicles on the left have this lockTurret [[2],true] in their init, with no delay. This should lock the Left Passenger 2 (MB 4WD) and Left Passenger 1 (Offroad) positions. By playing the mission and attempting to get into the vehicle in those positions, you can see that it doesn't work - the seats are still available and can be got into.

The vehicles on the right have the same command with a sleep 1 before it. By playing the mission and attempting to get into the vehicle in those positions, you can see that it does work - the seats are not available and can't be got into.

The Hunter GMG has the command with no delay, but its gunner seat cannot be got into - proving that the problem only affects a certain type of turret seat, not all.