TDD helps make your code better. In your case, it sounds like the key criterion was not that the code work well, but that it let you validate your belief in its business value, with the minimal possible time/money investment. I would agree TDD wasn't applicable, because it didn't provide the specific value you needed.
I would say, though, that once you reach the point where good code becomes important (and even startups face that need), TDD can help you write better code in less time with less effort. Please note the use of "can help"; IMHO neither TDD nor any other methodology will guarantee success.
I would say, though, that once you reach the point where good code becomes important (and even startups face that need), TDD can help you write better code in less time with less effort. Please note the use of "can help"; IMHO neither TDD nor any other methodology will guarantee success.