I briefly scanned the article. I am not that impressed by the result. I thought that an average smart person and particularly someone with a scientific background could reach that resalt in one or two months.

My experience. I played chess when I was between 13 and 16, at that time I was a good player but not a child prodigy, let's say one of the best players in my school. I have no idea of my ELO then, there was no internet in the early '80s. I have not played for roughly 40 years. I few years ago I started playing on lichess and my ELO is now around 1700 (with a peak of 1900). I don't know if chess.com and lichess have different ratings.