But you always see the difference between good and bad players during progression while the mechanics are still relevant. Once the teams start to outgear the content you're able to ignore the mechanics more and more. This is when the bad players will be able to just tunnel the boss and the difference between good and bad players becomes smaller.
If you take a good player and a bad player and ask them to attack a target dummy, then the difference in performance between them will be very small. So once the bosses become target dummies, the bad player is able to get very close to the good player.
And just to clarify, when I say a "bad player" it is still someone who are able to do mythic raiding but is just not performing very well during progression
TL;DR: Parses during progression matter. Parses after 20 kills don't matter. Because at that point the bosses are target dummies.