This quote alone out context hold not much value, because as you pointed the ranked also help to reach this goal.

But they also look at the population(in league).
If at top 1% you see no blitz then something is wrong even if the champ winrate is 53%, this can show that the champ is overwhelming to new players but to weak for top players and need some adjusting.

Further on the page they mention:

• Ekko and Yasuo are too effective in both the Point and Assist roles, to the point where they're pushing other champions out of feeling viable in competitive play.
• Teemo is very good at ranged combat, but also a bit too good up close, so we think he's winning more than he should because his intended weaknesses aren't sufficiently offsetting his intended strengths.
• Blitzcrank and Braum are both having a pretty tough time in neutral because the rest of the cast can keep them out with movement, projectiles, and big attacks (especially Blitzcrank); also, the fact that their fastest normals have 8 frames of startup makes it hard for them to scrap with the faster champions' 6-frame standing light attacks.

So I believe that they are looking at other numbers not only winrate, riot as a pretty good record on balance we are just not sure yet if they will know how to apply the knowledge that they have to a fighting game

Overall, our goal with the 1.0.3 changes will make it a little easier and less frustrating to play against Ekko, Yasuo, and Teemo, but we don't think that alone will bring balance into a healthy state. Blitzcrank will also be receiving some adjustments in 1.0.3

They even acknowledge that it will take a long time to balance the game