In the bolded section he seems to link our morality to God's moral nature.
Not exactly: he says that by doing good to offenders we
act like God, that is, we act in the way God acts.
We as image-bearers must bear some semblance to God's moral nature in order to be moral. So it is a fair argument to ask why God would demand something of us that is not fitting to his own moral nature. And God's moral nature is such that he is willing to forgive, but only actually does so when repentance is present.
Again I register my protest that speaking about God's moral nature often does nothing more than needlessly confuse us. Be that as it may, however, it has already been pointed out that the Bible itself registers a difference between God and men in the related area of vengeance. So even if it could normally be presumed that what God does is held out to us for imitation, we are forced to recognize that this is not always the case. And it is not hard to understand: in many respects a father is pleased when his children imitate him; but there are privileges, prerogatives, and responsibilities that he possesses which his child does not possess and should not try to imitate. Is a father a hypocrite when he puts his child to bed at eight while himself staying up until ten? The child does well to imitate his father
only in those points which are proposed for imitation. For us to imitate God without warrant is presumption rather than obedience; especially when it brings us to act as though sin were against us, and as though we had the right of releasing or retaining it. Our rule is God's command, not his providence and not his own actions. (Shall we drown the world in a flood? shall we command genocide? shall we punish our son in the place of the neighbor's boy?)
As I said before, every person is necessarily related to God, and to God as judge. But most of us are not judges, and we don't have any judicial relationships to other people that could not be otherwise. The argument from God's moral nature through man's image-bearing to one of us holding something against an individual until he expresses repentance takes no notice of these considerations. Until it accounts for them, it doesn't get off the ground.
Let's put it another way. In addition to the facts that everyone is related to God in a way that no one is related to us; that all sin is ultimately against God; that God is the moral governor of the world and we are not; that God is infinitely beyond us in dignity (and in everything else); that God has a right to require authoritatively what we cannot demand; in addition, I say, to all that is also the fact that God has the power to bestow what he requires. You contend that God demands repentance for reconciliation, and rightly so; but God also exalted Christ to give repentance. Now if you choose to imitate God in holding someone's offense against them, can you also imitate God in bestowing repentance on them? If not, then your putative imitation of God on this point is necessarily partial; and once that is admitted, the nerve of the argument is excised and disposed of.