There are often multiples applied when done with Rashi and Tosfos and Rishonim.
Rashi is ikar limud.
Otherwise true.
However, if he did it just Gemara Rashi, he would have finished even faster.
OT but if you learn something well, you will be able to do it quicker the next time.
So take a bachur of above average inteligence, and extreme but healthy hasmada, and a mehalach halimud that focuses on covering ground while getting stuff clear and not messing up your comprehesion of the sugya by premature addition of lamdus... You get a 24 year old who has finished shas with Tosfos multiple times, on top of the times just with Rashi.
Anyway, not sure what the shock is - People finish shas every year. how long do you think it takes them each day? For most of them its not even their ikar limud. a bachur sitting at a shtender for 12+ hours.... I just know I don't yet have that kind of hasmada nor that level of ahavas hatorah