Is this only a fan theory, or is there an official explanation?
No, this is not in any way, shape, or form an official explanation of Jar Jar's. This was entirely a fan-devised "crazy theory", which just happens to have an interesting amount of coincidental supporting evidence. The theory originated on reddit (where this kind of wild speculation is common and part of the regular entertainment amongst fans).
As far as this becoming "official" in canon, that's highly unlikely. In order for that to happen, Jar Jar would presumably have to appear in some subsequent canonical work. The theory even originally postulated that Jar Jar would
play a central role in The Force Awakens
However, as far as anyone associated with the movies has said, that's not going to happen. For example, according to producer Kathleen Kennedy:
“Jar Jar is definitely not in the movie,” she said to applause and cheers. src
I apologize for not committing Eps 1-3 to memory, but I seem to recall that JJB was a bit clumsy. Unless that theory postulates that his clumsiness was purposeful and all the trouble he caused due to his clumsiness was calculated to attempt to kill some of our heroes, I find it impossible to seriously consider the idea that a being that clumsy could be a Sith.