The difference is that there's some element of challenge to it, this is basically just the act of mashing the x button to do basic attacks. The condition of "before [x] boss" doesn't make it more difficult, it just makes it more tedious and time consuming. The most impressive thing about it is that they managed to commit to such a stupid idea for 500 hours of their life over a year.
It's the video game equivalent of your mom telling you you have to finish your homework before you can play with your friends, and trying to convince you you'll have more fun because you won't have to worry about getting it done. Then you get to college and realize "Oh, I can just do bits and pieces of the homework whenever I have spare time that I'm not actively doing something else and life is much more enjoyable without segregating my time into huge chunks of pure misery and chunks of enjoyment."
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