They explained it the very next chapter. He was born from a human and shinigami. I don't see a need to constantly point that out either when everyone else in Soul Society considers him human as well.
Chapter was ridiculously bad-ass, this Deicide arc is going out with a bang. Hated the story for a minute but Kubo's art is top notch, I'm really hoping for an Urahara monologue after everything is over where he retcons Aizen's bullshit with something that makes sense.
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