It's quite clear that Mad was referring to the soda, not the phrase, because Mad often used the logo for the soda (google it and you'll see it; note the long slan-like tendrils coming out of the northeast and southwest arms of the "X" in Moxie.) Wally Wood in particular would slap down the Moxie logo in the corner of various panels. (I just looked thru a bunch of my old Mad paperbacks in search of an example, but got distracted by Don Knuth's first publication, "The Potrezebie System of Measurements."