If that fails the interview, especially in a startup interview of all places, then that's a red flag on their end. Shows lack of pragmatism, poor communication

If that fails the interview, especially in a startup interview of all places, then that's a red flag on their end. Shows lack of pragmatism, poor communication and unwillingness to embrace lateral thinking. You came up with a working solution that fell outside the spectrum of what they were expecting. And they couldn't adapt their thinking/hiring process to that so more fool them. In a generous interpretation of the situation they weren't a good fit for you anyway.