The problem here is the lack of a thermal trap. True, you do see a trap in the plumbing sense. The injection riser piping does drop at least 18 inches below the primary loop before making the U-turn. But this piping geometry doesn't constitute a thermal trap. The reason is that the closely spaced tees that connect the injection risers to the primary loop are not at least 18 inches higher than the tees that connect to the distribution circuit.
The “U-tube” shape of the injection risers below the dashed line in Figure 2 is “buoyancy-balanced.” They do not create any unbalanced forces that would lead to buoyancy-induced heat migration. This is true for both the hot and cool U-tubes, even though the weight of hot water is less than that of cool water. It remains true regardless of how deep the U-shaped piping assembly is built.