A crazy theory, not meant to be taken seriously: What if he actually was Leonardo da Vinci? (After all, that was also a character assumed by Flint the Immortal, who could have taken up his 23rd century hobby of building androids in the 24th century again. Yes, Mc Coy stated that he was finally dying, but perhaps a periodic revisit to earth every 40 years or so would rejuvenate him again. Also, we don't actually see Soong die or hear anything about it after his private "goodbye" scene with Data so he may have faked that).