IronNat
On kickbacks, your elbow is next to your hip
On overhead extensions, your elbow area overhead
The shoulder is in a totally different position. And the long head crosses the shoulder joint.
They are not the same movements
I've been training for over 40 years. I know all of what you are saying.
The tricep kickback concentrates on the lateral head. Overhead pressing/extensions concentrate on the long head.
Both movements hit all three heads somewhat.
The tricep kickback is considered a finishing movement for the triceps. You would do it last after heavy pressing.
It is similar to doing tricep pushdowns for the lateral head. You should do them last as a finisher.
To say kickbacks do not develop the tricep is incorrect.
They were commonly done by old time bodybuilders and they worked.