Well,

it is exactly what the quote said:

The cost to the typical voter of voting in epistemically irrational ways is nearly zero. The cost of overcoming bias and epistemic irrationality is high. The psychological benefit of this irrationality is significant. Thus, voters demand a high amount of epistemic irrationality.

In the case of LW, voting irrationally has almost zero costs. You don't get penalized for voting wrongly(Incidentally I suggested trying to implement some measure of this kind and guess what... I was downvoted). The penalties are more indirect, like diminishing the amount of epistemically correct contributions.

So why would you assume that LW would be less prone to have this sort of problem?

The evidence suggests that the problem should actually be worse on LW, see1, 2.