take into account that the 'up' signal may never be received, for some reason.

Yeah, I found that strange in his spec, too. Most consumer infrared devices work like:

Button down signal.

Button held signal.
Button held signal.
Button held signal.
Button held signal.
(... etc until the button is released then it stops.)

There is no button released signal because it's infrared and therefore might never arrive. Since you need to code your receiving app to take that into account anyway, there's no point in having a button released signal.