I just tested it and under perfect conditions the PTAB AT bombs do around 1/4 to 1/3 HP damage on an Elefant or KT. What you say is just factually wrong. I think what happened in your test is that you used the regular IL2 bombing run instead of the PTAB one.
Idk what your "perfect conditions" are, but I think you just dont know how to use the ability. This test did so much damage it passed the threshold required for the main gun crit dice roll
You want to call it in at a slight angle, not perfectly parallel which is what I assume you were doing
This ability is absolutely phenomenal, and apparently it is one of the most underrated abilities in the game. Idk how this many people think it sucks. I mean the above test is over 900 damage AND a stun-lock AND a vision blind crit. What else do you want from an AT ability?