Garp want to beat Don in his best forte, physical and CoA, if he would use adv CoC like this chapter, he wouldn´t touch his head as he did in his clash with Don bro:
Adv CoC>not touching, he literally clash with him, adv CoC was not used here, so Garp legit didn´t used his full power.
Also Garp could choose a other body part to hit, but he hit on purpose his head, it was a challenge for Garp and in the end he won it.
He never go all out against Don.