It necessarily isn't since it's only available in a handful of countries. While there are methods to get it (APKs, VPN, alt accounts), they aren't really easy to perform so the mainstream public wouldn't use them. Also, if he is counting downloads only, the APKs would be out of the estimation.

The game was also blocked in several regions to avoid that people use it before being launches in their countries. Factoring all of this, the number is even more impressing.

User retention after a week?

GGOAT!!

"best user retention of all time!"

Not sure if the statement is flawed or mobile users move on from app to app pretty quickly.

Yeah I dont get it.

Mobile users move on within a day or two. 

Yeah, like the users of Candy Crush and Clash of Clans or any other popular mobile game.

Best user retention is determined 6 months down the line, not a few weeks.

so his point stands for the rest of the 99% mobile games that aren't popular.

Guess what, unpopular games on any other platform also don't hold interest of their users for long. So the point he is making is none.