It's a plothole if there is a gap in the logic of something and there are characters present that have been established as clever geniuses and they don't notice said gap.
The fact that there is an inconsistency in Hisoka's story is not a plot-hole. After all, his story is fabricated. The fact that there's a way to test it and neither Chrollo nor anyone else present thinks of it, given the intelligence that has been attributed to some of those characters, is one.
That would be all well and good, IF someone present had at least thought to bring up the obvious solution, and Hisoka had similarly refused.
But the fact that it wasn't brought up implies that either the writer himself didn't think of it, or he couldn't think of an excuse to get around it, so he simply didn't bring it up and hoped no one would notice.
Clearly it wasn't if several members of this board noticed it immediately and we're probably not nearly as intelligent as the writer wants us to believe Chrollo and some of the other spiders are.