Look specifically at the "Win-Rate % by Games Played", and then look ONLY at 125+ games. This represents Maokai mains, and is essentially the only stat that really matters, these are the people who have mastered Maokai and play him at his full potential.
That's oversimplifying a lot. Champions that are rarely played systematically have much, much higher numbers in this category, even if they are weak. Just saying "only this number matters" without analyzing any further would have us labeling Yorick, Galio and top-lane--Mordekaiser-top among the strongest champions in the game, and that fails even the most basic sanity check.
Azir (before he got nerfed) had a low base win-rate, but a super high 125+ game win-rate
Funnnily enough, that has never actually been true. His 125+ winrate has always been only slightly higher than the winrate for 0-5 games, and none of his winrates ever broke 50%.
Unless a champion is under 48%, there is no point of saying they are weak, and even then there are exceptions,
Exceptions work in both directions. Just as there are factors that can deflate a champion's winrate (such as them being hard), there are also tons of factors that can inflate winrates. Just saying that a 50% winrate = not weak is missing a ton of possible conditions that can influence winrate.
One more thing, Sejuani was broken even before Cinderhulk,
I know. That was exactly the point I was making. Riot and players were perfectly aware that she was very strong. I was making that point in response to you saying " remember how long it even took people to realize how strong she was, even after the cinderhulk changes?", which was inaccurate because people already knew it well before the Cinderhulk changes.