Indeed, the idea of 200 points in 16 days is more or less absurd, at least beyond beginner levels. In fact, looking at the author's Lichess profile I see no evidence of any improvement at all, to be blunt.
Improvement takes really a lot of work, far more than I think the author appreciates. Going from 1800 to 2000 in a year would be an excellent outcome and would require hundreds of hours of work on chess.
The author dismisses the one method that is arguably most recommended by strong players: studying GM games. Chess is about playing good moves, and studying the games of the world champions helps you learn what good moves look like. Studying GM games is hard work with little tangible reward (no dopamine rush from finishing a puzzle or playing a quick game) but it seems highly effective.
Another thing that stuck out to me: the author dismissed tactics puzzles based on what looks like 30 puzzles done on Lichess. That's nothing. I've done 7500 tactics on Chesstempo standard mode alone and it did a lot for my chess. It was basically the only thing I did to go from 1300 to 1600 or 1700 or so (over the board). Tactics aren't a panacea for chess improvement, but neither is anything else.
I have some smaller comments (for example: re: "If I want to get difficult puzzles correct, I don’t rely on any pattern recognition. I just spend a lot of time and mental energy explicitly calculating out every combination of moves. I’m really not learning much, most people can get the same puzzle correct if they just thought as hard as I." -- pattern recognition is the basis for calculation skill so it doesn't make sense to say you don't rely on it when calculating) but the main point is that chess is hard and improving takes a lot of hard work, far more than the author makes it seem.