Unfortunately, there are no "transferrable skills" that can be acquired on some other instrument and then applied to the theremin...
I would respectfully disagree with this statement for the following reason....
Any formal musical training over time helps to develop our brains...Oliver Sachs, the neurologist who wrote "The Man Who Mistook His Wife for a Hat" states that he can look at a human brain at autopsy and usually tell if the person was a trained musician based on the amount of development of the corpus callosum, which is (by my understanding), a sort of right brain to left brain connector. This is a physiological development that would enhance/affect any other communication skills, including all musical skills.
Clara Rockmore was a prodigious musical genius, who was formally trained from her youngest childhood, and who therefore had a lot of her brain circuits dedicated to music and fine motor control of her hands, so that she could play the violin. I would argue that this brain development made it possible for her to develop her playing skills on the theremin. She had no one to teach her how to play, but did quite well nonetheless...
I have devoted about about forty years of my life to playing reeds, so I believe there's a portion of my brain which has circuits devoted to the movement of my fingers changing pitch. I play theremin right handed, so when I fan open my right hand, with my pinky closest to the antenna, the pitch goes UP...This is also what happens when I open my right hand as I come up a scale on any of my reed instruments....I cede your point that there is no DIRECT connection, and that theremin is kind of out on an island of its own, but I would argue that any musician with formal training during their formative years has an advantage over any person with no musical training when attempting to learn the theremin...
I believe you are such a good theremin player partly because of your pre-existing musical neurological development, which is a result of ALL of your musical training, and partly because of your (who knows how many) hours of dedicated practice on theremin...