During the late 90s and early y2k pornography was still hard to come by for under age persons that wished to acquire or merely view porn. At this same time, Pokemon was at it's peak of popularity. It was assumed that any conversation mentioning the fad would go unnoticed by adults.
The phrase actually originated from the first label placed on the side of a stolen -- from a family friend-- homemade VHS tape with XXX programming on it. This and similar porn tapes could be placed among other non-adult VHS in the boys collection with out raising any suspicion.
Many pornographic release series stretch on higher and higher in volumes like Rocky movies, hence the phrase fits what it is.
"With all my homemade DBZ and actual Pokemon tapes, my parents will never find Pokemon 5."