At one time, decades ago, it used to! (Or another bus route). I've looked for the history on this and have been able to confirm it verbally with other folks my generation, have yet to find it as written history that can be linked. That tunnel has gone through successions of being paved and not, and different walkways or none at all at one time through there.
Your point on the need for better service to the subway is very valid to the point I'm making with extending the 504 to the Humber Loop. Of course there's been lots of talk of building an LRT along the waterfront to there, lol...but Toronto likes to talk a lot about the hypothesis of transit expansion.
The first impression of the 504 extended west to the Humber is one of absurdity, except travelling up Roncesvalles is already absurd compared to what the King Project is attempting to achieve. Roncy could be served just as well by the Queen Car, and many of the passengers on the 504 could continue to destinations further west by distributed off-loading onto the north-south routes. It's far from ideal, but better than what now exists. It would also render the extension of the King Pilot to the Queensway as yielding a much greater result, and in doing so, be an easier sell to Council.