I know it's become conventional wisdom for startups. But I'm not sure why it leads to success. There's plenty of examples of companies that did things that don't scale and are successful, but that's correlation. What's the causation? Why does doing unscalable things work?
You have to go beyond the title here. If you don't have product-market fit (as they say) and you're not communicating with customers (because that "doesn't scale") then you're only receiving deafening silence. You know nobody is using your product but you don't know why so you don't know what to improve. It's really "do things that help you learn from customers even if they don't scale".