Your moderator (read: me, but tired beyond reason) was operating on exactly 2 hours of rest. What I should have typed is "Please use Rude or Abusive for this. If you have any questions ask on meta." Lack of sleep was a factor here, oops, for that message.
PointThat not withstanding, though, I believe that such rants are valid use cases, in my opinion, for "Rude or Abusive" on the grounds that it doesn't warrant respectful discourse. I personally don't believe that 'ranting' falls within the "Be Nice" rule as rants target things with rudeness, and ultimately are intended to just complain in a non-nice way about things. Consider that "inappropriate for respectful discourse" is actually part of the flag reason:
Which is why I said you may wish to use that flag. However, when one of us operates on two hours of sleep, we often don't reply to flags properly.
(My other opinion: rants don't fall within the category of 'respectful discourse' either, whether it's using foul language, etc. or not.)