solo queue winrate doesn't mean anything
most of the strongest champions have surprisingly low winrates because people "copy the pros" and inevitably suck. look at shen, he has like a 40% winrate because retards pick him if he isn't banned, and proceed to suck terribly. but, shen is arguably one of the strongest champions in the game if played correctly.
sejuani is an okay jungler because her ganks are strong and she has great early-midgame utility. but, come lategame, she becomes an ult bot and then poses zero threat. sure you fire off your ult and stun some people for a few seconds, and then charge to their adc only to do pretty much no damage, get kited or straight up killed, and hope to god the rest of your team is competent enough to clean up the rest of their team. i guess she's also an average peeler, but there are better champs for peeling than sejuani.
reason she has such a high winrate in solo queue is that, not many people play her, and those who do can usually snowball lanes and get the rest of the team really far ahead.