To say that Christ became humanly impeccable after a human victory over sin in the wilderness at least acknowledges His impeccability from that time onward, but it does so for the wrong reason, and it also denies His impeccability before that time. To base Christ’s impeccability on a one-time victory at the human level in the wilderness is to end up with no impeccability at all, since all of us know that we can resist a particular sin at one point in our lives and yet commit that very same sin a few days or weeks later.
It is sometimes said that the angels in heaven who resisted the temptation of Lucifer became eternally holy after that time. This may or may not be true (some Bible scholars believe there was a second fall of holy angels), but even if the angels became eternally holy at that point, their holiness is not based on their own intrinsic impeccability. Instead, the angels are kept holy by the power of God. In all the universe, only God is intrinsically impeccable—totally nondependent upon any other being for maintaining personal holiness.