That's funny, because I had the exact opposite reaction. I was using Consolas, and when I turned on JetBrains Mono I thought "Ah! This is so much clearer!" Just goes to show you how subjective fonts are.
But readability and legibility can be measured. It's unfortunate that Jetbrains makes a bunch of claims without supporting any of them with results surveys/studies, because if right, that would translate to other font designs as well.