If William somehow had a legitimate daughter before the 28th October 2011 they would have started the process to change the law to cover that daughter.
If William had an illegitimate daughter it wouldn't matter as illegitimate children can't inherit the throne.
In 1981/2, when Diana was expecting William, there was talk of changing the rules to allow a first born daughter to inherit. However, when William was born a boy they decided to do nothing.
It wasn't until the possibility of a new generation of children being born, and thus the possibility of a first born daughter being replaced by a younger brother, rared its head again, after William married, that the relevant leaders decided it was time to do something. It took them nearly 3.5 years however so even though the agreement had been made until all the realms had passed the law and it had been gazetted in the UK - in March, 2015, it wasn't actually the law.