That part was apparently added a bit later, when he posted What Evidence Filtered Evidence.

It cautions people against interpreting the entire preceding post in this literal way. Presumably it was added because people did interpret it so, and gRR's reading is not novel or unique.

Of course this reading is wrong - as it applies to reality, and as a description of Eliezer's beliefs. But it's right - as it applies to the post: it is a plausible literal meaning. It wasn't the intention of the writer, but if some people understand it this way, then it's the text's fault (so to speak), no the readers'. There is no "true" literal meaning to a text other than what people understand from it.