Well he was on Graham Norton's radio show this morning and said he wasn't allowed to talk about it as it got him into trouble. Don't think the readers for DOTD are top-secret info.
I worked with marketing copy for the fifth one recently. Title, plot, and reader were all, at the time, TBA. Yes, there was a big splash of information on the first, but after that there's less and less information from AudioGo. So, no, it doesn't surprise me one iota that people would ask Barrowman to dial back his enthusiasm if he's the reader of the ninth Doctor audio when they're being less than forthcoming about Destiny of the Doctor