The reason it is nonsense is that you do not understand very basic mathematics.
I quote you again:-
'Change of longitude (dīlong) is an angle measured in minutes at the Equator, and departure is a distance in nautical miles elsewhere. More technically, departure is the distance made good in an E-W direction along a rhumb line.'
More technically your definition is again totally incorrect.
This is a total misunderstanding of what 'departure' actually is.
Departure = Dlong * cos lat at ANY latitude.
At the poles cos 90 = 0 so departure = 0
At the equator cos 0 = 1 so departure = change in longitude
If you want nautical miles the formula becomes 60*dlong*cos lat
Why do you have such difficulty with such a simple school formula when you are are briefing others with your incorrect information?