As I stated, these are non-scientific sources. Since atmospheric pressure is not linear with altitude, I suspect that the stated relationship is a simplification of the actual relationship based upon atmospheric pressure versus altitude for lower altitudes since this is where the majority of people live and brew. If you search for some more scholarly papers, you will likely find a more appropriate model which would better explain the relationship between utilization, boiling temperature of wort and altitude.
IMHO, the IBU number gives us something to shoot for for consistency within our own process. The calculated number and its relationship to perceived bitterness is pretty fuzzy to begin with. When you then throw in lower boiling temperatures and higher rate of volatilization of hop oils at much higher altitudes, the relationship cannot be a simple linear function as referenced by an excerpt from "Whats Your IBU" by Michael Hall, Ph.D.:
Boiling-point temperature: The isomerization reaction rate depends on temperature, so the boiling-point temperature at your elevation can make a big difference. At my elevation, 7,365 feet, water boils at 198 degrees F (92 degrees C) instead of 212 degrees F (100 degrees C). Garetz gives a correction factor for this effect:
Fbp = 1/(1 + Eft / 27500), where Fbp is effective boiling point in degrees F and Eft is elevation in feet.