To get into the top 10 apps list, you almost have to buy fake sales. It's the status quo now.
That said, a lone indie dev from Vietnam could never afford to play that game. It makes much more sense that his game just "went viral".
I know that my partner heard about it via word of mouth, and I heard about it because someone was swearing at their phone in the office lunch room while playing it
It's free, it's extremely simple, and it's fun in the way that games used to be in the 80's and 90's - simple, fair, extremely hard challenges.
The Mario-inspired graphics are nostalgic, the difficulty of the gameplay is nostalgic too. If you're not a "core gamer", so that these things aren't nostalgic for you, then they're novel instead!