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I don't believe it is perpetual motion, because you are not obligated to reel it back in against the same force that took it out. But it is a silly and fidgety way of generating electricity.
Using 2 kites in opposition they could create some sort of short frequency oscillation on the cable and engineer something to run off that at ground level.
However, if they are planning to operate at sea I could see a number of materials science challenges for which the engineering costs might outweigh the energy creation benefits.