It already should be clear from the existing text that the hours in the day
are numbered 00, 01, 02, ..., 23, so that 00 is the first hour of the
calendar day and 23 is the last.
My argument is that that isn't clear, actually. That's the assumption we've all enjoyed, but it's evident that some prefer to count differently (for whatever reasons).
One can use so-called Ethiopian timekeeping in any timezone;
should we set up an Ethiopian variant for every current tzdb identifier,
for the benefit of Ethopians sojourning in other locations?
Not at all. Rather, by explicitly specifying that tz assumes hours are counted from "midnight", we emphasize that those sorts of adjustments are, in fact, outside our scope.
I won't be bothered if it's left out, but I do think it helps to be clear here.