Archer and Phlox alone choose for this world.
They simply didn't commit their involvement beyond the point of no return. There's still plenty of time for a cure to be found by other parties, for other powers to intervene, for Earth's government to overrule Archer's decision of what sort of aid to provide, etc.
By easing their suffering, perhaps Archer even improves their chances of finding a cure on their own.
Had Archer given them the cure, that
would have been an irrevocable choice.