It is very unlikely that there is light throughout the entire length of the tunnel, but it is pretty likely that there is some light at the entrances of the tunnel. The point, I suppose, is that the shadow is cast by the ground that is hit by light on the side opposite that of the shadow for much of the tunnel's length. Even if both ends were plugged, there would still be a shadow within the tunnel, but a Nara wouldn't be able to use it unless they were either within the tunnel or forced their shadow beneath that of the entrance pluggers.
And Eric could have told Neala that she can use tunnel shadows.
It's kind of been my thing for a few years now.