I think you have confused the term "Jeep," the amorphous slang term:
1) From the Saturday Evening Post of 16 July 1938:
For a foolish or inexperienced person.
2) From a 1941 article in Jimmy Cannon’s Nobody Asked Me:
A recruit or basic trainee.
3) The New York Times July 31, 1938.
In this case Jeep is being used to refer to a tank, not the M38.
and how this particular vehicle and the character "JEEP" came together. Irving Hausmann, the civilian Willys-Overland engineer, the very same test-driver who replied to Hilyer it's a "Jeep," was not referring to what you beleive to be the inspiration for the term. He was testing the vehicle at Camp Holabird, MD where other soldiers, who had seen the 1940 "Popeye Presents Eugene the Jeep," and had christened the vehicle "Jeep" because it could go anywhere just like the cartoon character. Prior to this, the prototype M38's were called "Combat Car", "Reconnaissance Car", "Bantam", "Quad", "Peep", "Pygmy" and "Blitz Buggy", but there is nothing about any referance to it being called "Jeep" until after the cartoon and film short.
During this period Disney characters, Warner Brothers cartoons, and Max Fleischer's Popeye had become very prominent in U.S. military culture. This vehicle was loved by those who helped develop it (these were not WWI veterans) and their affection for it was demonstrated by naming it after a beloved cartoon figure. For you to dismiss this connection with some baseless commentary about the ignorance of civilians and their dependence on pop culture regarding military jargon is what "almost certainly isn't true." Apparently many G.I.'s, including Gunny Lee Ermey, of The History Channel's "Mail Call," also subscribe to the "Eugene The Jeep" connection and poo poo your "coincidental" theory.
From your own wiki source:
Many, including Ermey, suggest that soldiers at the time were so impressed with the new vehicles that they informally named it after Eugene the Jeep, a character in the Popeye cartoons that "could go anywhere."
http://en.wikipedia.org/wiki/Jeep