The androids were actually invented by an earlier civilization. Dr. Korby is a lone genius exclusively in the "stumbled upon an artifact" sense -- in fact, even before the androids, his fame was as a "medical archaeologist", i.e., translating ancient lost medical knowledge for modern use. He made a career out of stumbling on other people's inventions, apparently.

Another lone genius from TOS: the prison warden/reformer, Dr. Tristan Adams, who had supposedly made prisons a paradise for rehabilitation, but had Gone Too Far and had built a mind-control ray for the tough cases (and anyone else Who Got In His Way).

TOS doesn't seem to like Lone Geniuses; they appear to fall in a similar category with "humans who suddenly get godlike superpowers". The recommended action for godlike humans is "kill them now while you still can; even if you want to try to live peacefully with them, you can't." There isn't the same urgency around killing Lone Geniuses (but y'know how Lone Geniuses are, they eventually get around to trying to kill you, and then you've got to kill them).