I'm not even sure whether the spirit of those replies is mostly to criticise the argument by showing it is absurd (on the premise that index investing is a known good choice), or whether they aim to criticise index investing itself (on the premise that the argument is good).
Maybe I am missing some context. Has Paul Graham strongly supported index funds, and so are these replies some kind of "gotcha"? Or is there some general disdain for index funds in some circles, and so are they just taking the opportunity to disparage the value of index funds by using this argument?