Seriously better not be calling me immature too, but w/e...
Its confirmed Nishi was around for a year before the series started, he came in the same hunt as Izumi's first time. This was all in the Gantz Minus Novel, where he witnessed Ooki leave via the 100pt menu and later in the time gap before the series, Izumi too.
Therefore he knew about the three options and decided to be free first. I expect he learned about Katastrophe in the time gap between the Oni and Nuri hunt, thus changing what he would select.
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