flufI wonder if these players really did improve faster than their peers or just knew the right people
It is a little of both to be honest. Popularity has its perks.
Being friends with the right players and playing as part of their circle jerk in pugs could make you a stronger player over time since you are constantly playing against skilled players.
On the other hand, there are a lot of players who are very overrated simply because they are part of said circle jerk.