Since Brambleclaw never stopped visiting his father, he and Squirrelflight never made up so he was never her mate. Squirrelflight became mates with Ashfur instead. Since he was her mate, he never threatened to kill Leafpool's kits in front of her, so she did not reveal her. (he finds out later which is why he turns against her and Firestar to side with the dark forest cats) Needless to say, Hollyleaf never kills Ashfur, never reveals the secret at the gathering, and doesn't get trapped in the tunnel.
Hollyleaf lives a happy life with her clan and was expecting kits.
Due to Brambleclaw's treachery, the battle ended much quicker and many cats, including Hollyleaf, did not die. However at this point, Hollyleaf does get trapped in the tunnel.