I seem to recall Musk specifically said it's not an evacuated tunnel. However, air leaks aren't a huge problem. You just need them to be slower than the pumps that run to maintain low-pressure; since the flow into the tunnel from leaks will increase as the pressure drops in the tunnel, you will reach an equilibrium at some point. For a really leaky tunnel that might be near 1atm; for a well-sealed tunnel it will be near 0atm; a practical transportation tunnel will balance construction and maintenance costs against the diminishing returns of lower pressures.