I feel like at that point the implication is more that they know when to pick her where she'll have an easy time succeeding — not just in terms of the exact team compositions, but who they're playing with and who they're playing against. In Challenger, you have the luxury(?) of basically playing with same people a bunch of times, so you know how they tend to play, what their weak points and strengths are, what they're likely or unlikely to pick (see: Annie Bot almost always wants Annie), etc. tl;dr Challenger is kinda screwy.
And, for what it's worth, I can understand your frustration, but I still think it sounds more like it's coming from personal difficulties handling her, and less from Janna herself being overpowered. And that's perfectly fine if it's a personal challenge as well, because her kit pushes a lot of points that can be difficult to adjust to, especially from the actual Janna one-tricks that are pushing her skill ceiling as hard as they can — predicting her Q direction, knowing when to disengage a fight after drawing out things like her ultimate vs. when to dive back in after she uses it, or baiting out her E with one thing so you can use something stronger once it's on cooldown.