Janna isn't being buffed. If she uses her W, she now has the potential to be the slowest champion on her team depending on the base MS of allied champions. Janna's Q was never a good poke tool because of the mana cost, the slow projectile speed, and now the narrow width. W was her main poke tool against low poke lanes, but now it will almost 100% of the time be used defensively as is the case with Q. She can help people flee and get back to lane faster compared to before. I would say it's a power shift to being even more of a defensive late game champion that tries to prolong the match and hope for enemy mistakes.