My interpretation of the term "Japanese Summilux" is that it is an f1.4 lens: Its mainly about maximum aperture, which has been a competitive endeavor among lens manufacturers. Who refers to their Leica Summilux 50mm as "My Leica 50/1.4"? Most Leica owners refer to the lens as the "50 lux" or "50 summilux". Per Leica's choice, the terms summilux, summicron, etc....refer to maximum aperture combined with a Summi- from the Greek "summus" or "summum" for "highest" to connote quality. It is reasonable to compare the Canon 50mm f1.4 lens to the Leica Summilux 50mm f1.4 with the term "Japanese Summilux"
IQ-wise, I'd agree the Canon lens is excellent. Price-wise.....thats another story, but there are those who value the "cachet" of the Leica label, and consider any substitute or comparision unworthy.
Personally, I do not view the term "Japanese Summilux" as pejorative. Rather, it connotes a 50mm f1.4 lens that also happens to produce excellent images.