I don't think it's even possible to transfer shark cards so I wouldn't bet on that explanation being right, but it seems near impossible for any video game to sell this much without smartphones and without being PC focused.
A constant 1/3 attach ratio seems impossible. That should be decreasing as more games release on PS4/XB1.
As PS4 gets more and more system sellers and just an overall stronger library, more and more people will be buying the system for those games instead of GTAV.Zhuge did analysis of it around a year ago, and it won't have changed much now. No matter what, GTAV sales very closely match PS4/ Xb1 sales month on month. And GTAV software sells around 1/3 of those HW most months. PS4 is getting a bigger library, so people are picking up more games when they first get PS4s, but roughly the same proportion of people (around 1/3) are still picking up GTA V as well. Its just a game that a lot of people think they need to have on their new console, and that rate isn't decreasing notably at all.
It is extremely strange, but that's just the way it is. I think after Red Dead, GTA V will finally start dropping off this rate as some people choose Red Dead instead.