Tokio is also an exonym for 東京 – in many languages other than English. While the spelling overlaps with how 「ときお」 (whatever that means) would be romanized, the

Tokio is also an exonym for 東京 – in many languages other than English. While the spelling overlaps with how 「ときお」 (whatever that means) would be romanized, the author himself told the name got its inspiration from the metropolis of Tokyo. Edit: By the way, your idea of "unambiguously" encoding Japanese doesn't hold water. Written Japanese is written in a mixture of kanji and kana, and no romanization system does round-trip the kanji. You could say that (good) romanization systems unambiguously encode the pronunciation, but that isn't true either: in addition to the fact that encoding "pronunciation" of written language is problematic in principle, there are the pitch accents that aren't captured by kana. But we're digressing. If you give up the "unambiguous", "tokyo" is a perfectly good romanization for 東京.