The stuff about an addition million unit order is conjecture and I think misrepresents what's in the actual press releases. Read for yoursef:
So on December 1, SE says that (Shipments + Digital Sales) has passed 5 million.
On January 9, they say that (Shipments + Digital Sales) have passed 6 million.
It's possible that 40 days of digital sales is driving the growth while just seeing a marginal increse in new shipments.
Honestly it's hard for me to believe that a game that has fluctuated between $35 and $50 new on Amazon and other retailers since a week after launch, and that GameStop was aggressively offering trade in bonuses for, is just burning through shipments. My gut (and brain) still says that it is a lukewarm reviewed JRPG released in 2016.