The author is delusional. That's exactly what 8th, or any other grade, is. Get enough points, proceed to the next grade. Score high enough on the leader board, proceed to the college you want.
Author: "Gamification is bad when applied to eighth grade."
I kept looking for the support for that claim, didn't really find any. The IPI study was tossed in, but it dealt with a tangentially related educational program, and seemed to endorse methods focused education over memorization of answers. This article was attacking process focused aspects of Khan, so I'm not sure IPI was the best study to cite.
It's a drawn out article, and light on evidence. I wouldn't look to the author for advice on teaching others critical thinking.
Gamification of education would be wonderful. The thing is, it wouldn't consist of adding things like leaderboards and achievements. As people have pointed out, we already have those. Gamification should really be about the return of play to the primary role in learning.
That would improve on the current system, which passes failing students along as well, right up until they hit university and find out they need a few years of remedial classes.
You can't just take something people don't like and don't want to do, and stick in arbitrary "levels" and have that motivate them. The levels have to represent some achievement that is desirable to the person.