I heard this being discussed and some unknown said that John Lennon liked to pull peoples legs. This could have been his own little joke.
It’s actually hard to ever really know. He did also say at one time though that he preferred the Tories to the Labours because they “let me keep my money.”
So who knows? It almost seems it would have been a natural evolution for a guy like him...in the 60s he was suspicious of “The Man” who, at the time, was regarded as a war mongering capitalist pig. However, by the late 70s it was clear that “the Man” was really the entrenched leftism that had overtaken the US government.