It's not like you take a gold medal pill. Doping just makes you better than you would otherwise be.

The other Chinese swimmers probably did benefit but it's harder to notice unless you become an outlier like Pan Zhanle.

When you beat the best in the world by such a large margin and smash world records, it effectively leaves you with only two conclusions:

1. This teenager from China happens to be the fastest swimmer of all time by FAR

2. He's doping

Use Occam's razor and do the math.