When I first saw the original post, I honestly believed it had to be an April Fool gag. 

Like Greg, and 99% of other people, I find RPN vastly harder to parse. I have also written or edited Postscript by hand, but much more small hobbyist level than a real work need. It's crazy difficult.

 I confusedly accept or stipulate that John is in that <1% who process this differently.

But the greater difficulty of RPN isn't only my greater familiarity with infix, it's also objectively measurable. The example John gives makes infix capriciously worse by omitting normal spacing (while keeping them by requirement for postfix). But let's bracket that.

A reasonable measure of difficulty is simply to count the number of eye movements required to parse each version. In John's example, or any of similar moderate complexity, postfix requires 1.5-2x as many eye movements as infix. You can measure this in a manner similar to Levenshtein distance, as a collection of small atomic actions.

It's not so much that moving eyeballs is difficult for most people. Rather, every such movement is a very slight mental pause to redirect attention.

Of course, you never need to do anything other than scan left to right if ALL you are trying to understand is the final result. But that's not what we do when reading computer code. Rather, we want to understand the WHY and the WHAT of an expression as a whole.