One man's meat is another man's poison

The qualification “as much as you properly can” is needed in case one faced a situation where one could promote the flourishing of one person (say Peter) only by withholding the true flourishing of another (say John), or by losing some other good that was even greater in value. I do not believe, however, that God ever faces a situation in which he can promote the true flourishing of one person only by withholding the flourishing of another, nor do I think he is faced with a choice where he might have other goals that are inconsistent with promoting true flourishing. For the true flourishing of all persons is a right relationship with God, so given God’s almighty power and wisdom, he does not have to choose between promoting the true flourishing of Peter, say, instead of John. He can promote the true flourishing of both. Jerry Walls, Does God Love Everyone? What’s Wrong With Calvinism, 30.

Consider a counterexample:

Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him (Gen 4:8). 

And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him” (Gen 4:25).

Seth is the replacement child for Abel. If Abel hadn't been murdered, Seth wouldn't exist. Therefore, Seth flourishes at the expense of Abel.

What is more, if Abel hadn't been murdered, he'd presumably father kids of his own. But he died before he had that chance. Hence, Seth flourishes at the expense of Abel's would-be children, grandchildren, great-grandchildren, &c.

This doesn't necessary mean Adam and Eve would not have had a third child unless Abel was murdered. But conception is a matter of timing. Throw off the timing, even by a few minutes, and a different child will be conceived. 

We could multiple examples. Because Joseph winds up in Egypt, he marries an Egyptian and has kids by her. Had he remained in Canaan, he'd marry a Jewess and have children by her. The flourishing of his Egyptian descendants comes at the cost of the alternate timeline. 

To recap:

I do not believe, however, that God ever faces a situation in which he can promote the true flourishing of one person only by withholding the flourishing of another, nor do I think he is faced with a choice where he might have other goals that are inconsistent with promoting true flourishing. For the true flourishing of all persons is a right relationship with God, so given God’s almighty power and wisdom, he does not have to choose between promoting the true flourishing of Peter, say, instead of John. He can promote the true flourishing of both.

Jerry has a bad habit of asserting whatever he needs for his theory to be true. 

In freewill theism, moreover, there are sometimes insurmountable obstacles to God getting his way, for human freedom can thwart divine preferences. Jerry admits that in other contexts.