It's a rational explanation that just happens to be ... wrong.
Cancel/OK suggests Cancelling first, OK'eying second. Whether your eye has already moved on from Cancel is secondary to the fact that you brain got implanted with Cancel as a primary option.
Unless you're trained to start with the right most button (something the article ignores).
Also, positioning isn't the only factor - you can also highlight the primary action with colour and texture, which I would guess is far more effective at implanting a button as the primary option than positioning. Most OS's do this as a part of their keyboard accessibility.
Cancel/OK suggests Cancelling first, OK'eying second. Whether your eye has already moved on from Cancel is secondary to the fact that you brain got implanted with Cancel as a primary option.