Forgive me if I'm missing something but isn't most of the AI debt based on fixed rates so would be unaffected by a rise in interest rates?

Technically, yes, except that most of the proposed AI buildout comes in the future, and those bonds have not yet come to market. (Nor will they until the companies are ready to build.) So in a real sense, the future of AI is/will be dependent on future rates, but yes, what’s done is done.

Note that the big expenses can be borne by the hyper scalers, largely with cash (although they are doing it with debt), but the players at the periphery also need financing to make things happen, so the construction company actually doing the build, the utility adding power capability, the people fabricating the radiators and fans for cooling and all the other “supporting cast” also need to borrow, and they do not have the cash flows that banks or bond investors trust, so they will have to use variable rate bonds or loans.