Uhm, you mean about high-volume training, don't you? You're saying you used to do that, then went to HIT, correct? I agree, I don't think you need that much volume to grow. Especially not 15 sets for a small muscle like the biceps. Still, if it works for them, it works. No one cares about how you got there. Do you see them asking at a bodybuilding competition "Now, you trained how?" "Uhm, high-volume. About 20 sets per bodypart" "Disqualified!". If that were the case, well, that'd just be fucked up and some people would have bullets through their heads right about now.
The results are all that matter.