The sejuani buff on the pbe does not address any of her main issues
As somebody that desperately wants to be able to play Sejuani again (and actually feel good about it instead of guilty that I am not picking another better jungler), I can't help but feel like the last thing Sejuani needed to be competitive again was more access to slows.
Her problems right now are mainly she is a late game champ in an early game meta. Her power spike comes on extremely late compared to other tanks and she can neither farm very effectively nor gank particularly well. Champions can easily counterjungle her and set her power spike back even farther if they wanted to (which almost every good jungler does).
She could be buffed in her clear speed like Shyvana was with her burnout, so that she can basically pull a Warwick and just concentrate on getting to 6 to finally be able to use her ultimate. Other proper alternatives would be to bring her flail of the northern winds damage back up to 8 percent of her health instead of 6 (it was 12 during the cinderhulk era) or increase the armor on her passive to at least keep her alive for counterjungling encounters so she can leave. Adding this extra ability to her permafrost will do almost nothing for her, as the AP ratios on it are terrible and the extra slow she will provide seems extremely situational.