I don't agree with them needing no direct work. It all depends on what your goals are. What is need? If you want maximum hypertophy in your arms, some direct work is needed IMO. Yes your tri/bi get hit during chest/back day. Do they get hit with the same tension as they do with skullcrushers/curls etc? Hell no.
I think it's advisable to hit your bi/tri when they're fresh once every couple weeks if your goal is hypertrophy in your arms. I do the chest/tri, back/bi routine also BUT every once every couple weeks I also do a bi/tri day a few days apart from a chest/tri and back/bi workout and I will either do little or very light direct work for the bis/tris that same week when they are worked with chest and back.
The problem is hitting them hard without overtraining them.