Yes, TDD seems to involve having a decent idea of the requirements (very un-agile). Many projects are making it up as they go along and iterating to a reasonably correct outcome.
It means having a clear idea about the requirement you're implementing at that point in time (not un-agile). It doesn't mean that you have to create a big set of requirements up front (un-agile). You can just pick a story, convert it into a test and implement the code, rinse and repeat.
Writing the test also gives you a second chance to think deeply about the requirement and fix any problems with it before wasting time implementing the wrong thing.
