There isn't enough people playing the game, matchmaking isn't proposing balanced matches due to lack of players available.
This is true also for the top 10%. You do not improve this effect by shrinking the player base you look at even further to the top 10%.
Below elite playing ranks most of the game aren't balanced and the faction played has little impact on the game outcome.
I agree that high level play reflects better the strengths and weaknesses of the respective faction.
In contrast, if you're an average player at (for the sake of the example) 50%, you have way higher chances of getting a balanced match than being at the edges of the skill curve due to the fact that you can get an opponent slightly above AND below your skill level.
High skill players get obviously more often matched with lower skill players, low skill players get more often matched with higher skill players.
The question is if the game should be balanced around top tier play or somewhat the main player base, but that would heavily derail the topic.
We have to live with the fact that different skill levels play with slightly different metas. Units like the Brummbär require heavy micro to be effective but die more easily in low skill level. Is the unit OP? Is it UP? There's no easy way to say that, but balancing it only according to the top players that can give the unit the high micro it needs will basically kick out the main player base.
I agree with the general conclusion that low skill data is not really representative of the whole game, but first the argument given does not make sense to me as well as the conclusion that only top tier play shows game balance while 90% of the games played are purely luck is also bullshit.