I think he's saying that the Power 5 weapons became much more available and the population began to decline for a long while as a result. When Mod 6 was released, population spiked up again for a short while.
Osiris appears to associate crafting with an increase in population in Mod 6, and is using that association to justify his belief that the Power 5 dilution caused the population drop.
Personally, I'm quite confused by his "logic."