The two of you have performed a limited experiment where your site was the control. Of course your soil may be different as may be water management but it's interesting that he got much more vigorous growth.

New apples are usually transplanted 2-year trees that suffer root damage when being dug up and therefore don't tend to respond the first season to fertilizer applications although they may grow faster the second season as a result of applications the first. The only consistent advice I read about fertilizing these trees is don't put quick release fertilizers in the planting hole.

I wouldn't describe a rapidly growing new tree as "running amuck" unless maybe it was my neighbors tree and was planted at the same time as my runted one. Then I'd probably do exactly as the neighbor did to try to duplicate those results. If it runs amuck then you should pinch prune it to get it running right.

Anyway, I suggest the MG if that's the fertilizer of choice, although any quick release nitrogen would likely have the same affect at same rates.