I would say that lack of information plays a part in this - IOMTT.com, Audi.co.uk, etc all only mention the TTS in conjunction with the TT races. The inference from the press releases is that the Audi TT will be the official car - if it isn't, the press release could've been worded better.

It now appears, from your reply, that the TTS is only being debuted in the IOM, and that A4s are being used as course cars - that makes a lot more sense than the press releases implied.