Usually, when no Defending units remain in the Battle hex because they have all been eliminated (except due to a Stalemate result, Defender can Press (step loss) result, or the application of A Bridge (or Beach) Too Far (...)
I think this rule ([328.2.1]) is only included in the full version of the ETO manual ; I don't see it in the original TITE collection or in this one for TITE 1A.
W44