No, since they use an integer (Z) as the denominator. So their representation support having -1 (i.e 0) as the denominator.

Yes, this is mostly a leftover from initial versions that used a natural number as denominator. It doesn't seem to make a noticeable difference in performance though, since increments are a very basic operation.

I think leaving this in the article makes the non-zero denominator more explicit. It also allows easier adoption to other numeral systems :)