[Joe J.]

My guess is that their intent was to define the tunnel walls and ceiling as part of the tower not necessarily the space they confine.

Otherwise would it not have been easier to simply say the carpet under the tunnel IS part of the tower or red cannot drive through blue's tunnel during the finale?

I think that because they say that the carpet under the tunnel is not considered part of the tower that their intent was not to say red cannot go through blue's tunnel but red can't touch the side walls or ceiling while doing so.