I'm not so concerned with the state versus prep allocation as I have it on good authority that they are considered as two separate pools to which a certain number of places are allocated (perhaps 50/50 in some schools as EITEL has mentioned.
I do think, however that those extra weeks of preparation for the ISEB could have meant those state school children taking the ISEB later were at an advantage. Personally, I was happy that DD sat hers early as one of her schools required it and it gave us some breathing space over Christmas. However I know of DC who were working through Atom and GL papers from October to early Jan and spending many hours with tutors over the Christmas holidays.
We totally bought into the idea that a child can't prepare for the ISEB but are now persuaded otherwise. If you had the money for online subscriptions and tutors who could help you adjust to the new format, I believe that made a big difference. The uneven playing field this year is within the state school applications in my view, not state versus prep.
It is what it is, however and I certainly don't envy the admissions departments. Many have interviewed all candidates this year, so hopefully that will give them additional information to go on.