TDD/BDD assumes you know the problem and are coding to create a solution. In startups, however, you do not know the problem.
I think this is where these posts on HN always end up talking at cross-purposes. "Startup" can mean many different stages. I definitely agree with this. It's one of those things where when I think "startup" I think "they've got a problem, they're solving it" whereas some people read it at that very early stage instead. TDD doesn't suit that stage, because, like you said, you don't even have an idea of what success looks like.
I think this is where these posts on HN always end up talking at cross-purposes. "Startup" can mean many different stages. I definitely agree with this. It's one of those things where when I think "startup" I think "they've got a problem, they're solving it" whereas some people read it at that very early stage instead. TDD doesn't suit that stage, because, like you said, you don't even have an idea of what success looks like.