Can I ask you something honestly? Is she had thanked DD or if she had talked about his music’s production without been asked wouldn’t you’ve taken it as a confirmation of them being together?

The comparison seem just impossible here. If she had acknowledged David, she would have known the consequences behind it. She can’t just drop David’s name on something not related to XF and expect people to not freak out. If he had helped her with the book, it would have been known beforehand, she would have added a few words to explain as to what he was acknowledged for.
I’m sorry but since the beginning of this rumor, I keep getting asks comparing David and Peter Morgan, and I keep reading things like “if it was David, they would take it as a confirmation blah blah blah”. But actually, it’s just impossible for me to wrap my mind on this comparison. Her relationship with Peter Morgan, whatever it is, will never be something to be compared to the one she has with David, either privately nor publicly. I don’t know how anyone being in this fandom and knowing Gillian and David can expect us to have the same reaction towards what she does with David and what she does with anyone else.