Supplementary training is just that, supplementary and should NOT take away from any direct combat work because it does NOT take the place of it.
That is like saying that weight training shouldn't be done because a fighter could be fighting training fighting which is directly more applicable.
IH and IB work is done by fighters on a regular basis, they just don't call it that.
Whenever a boxer trains to take a shot, it is IB work and everything a boxer hist the bag its IH work.
It just a TYPE of IB and IH work.
That TCMA use a specific type of SUPPLEMENTARY IH work that goes beyond the standard is something that is a matter of tradition for them.
Can a modern pro fighter get benefits from it? sure, but there is a time frame that MUST be applied to PRO fighters and where do they have the time to ADD an extra 30-45 min minimum? typically they don't.
They ALREADY get A LOT of fist conditioning, far more than the recreational MA.
A boxer doing 15 rounds on a HB, every day gets all the "IH" work he needs.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !