各位老师好。

剑桥雅思12的第二套的阅读题中的文章The Lost City里有这么一个句子：

First, as he climbs up the hill, he describes the ever-present possibility of deadly snakes, ‘capable of making considerable springs when in pursuit of their prey’, not that he sees any.

请问最后一小句“not that he sees any”是什么成分？是deadly snakes的同位语，或是独立结构，或是其它？

这句话很好理解，但总觉得有些怪。比如为什么不说成“that he doesn't see any”？