No, it's not an affirmative defense. It's a real assertion. It is an argument used when there is no consideration. It is an argument that is an ALTERNATIVE to consideration.
I gave a similar example above (in response to
@AtlAtty . So if Hugh HathNoCock promised to give $15 million to build a building, and then halfway through construction he reneges on his promise, UiF might ask a court to use promissory estoppel to compel HathNoCock to make the promised payments. So even though the promise between HathNoCock and UF was a gratuitious promise, and lacked the mutual consideration to be a contract, the University then detrimentally relied on this promise to enter into a separate contract with a builder to construct the $15 million building. And the argument to the court is for the court to compel HathNoCock to make good on his promise.
That's all.
I'm not saying it will definitely succeed, and there are certainly massive PR risks. But that's the legal strategy in a nutshell. Again, it's fun for conversation, but it's fairly unlikely to actually happen.