Board Thread:Questions and Answers/@comment-10797017-20130627100621/@comment-26420452-20130715164336

Sōsuke Aizen is more than just creator of the Hogyōku and an immortal being. He is a master of all four aspects of Zankensoki and Kyōka Suigetsu was one monstruously strong Zanpakutō (and apparently, he still has some of its power left, he was able to put Yhwach under hypnosis mentally).

Uryū is not the only Gemischt to survive the Auswählen. Technically, Ichigo, Karin and Yuzu are Gemischt too.