If you put two people on the exact same problem, sure. But, software systems are made up of LOTS of problems, a large amount of which can be solved in parallel.
That's why it's a platitude. It gets bantered about by developers as an explanation about why we just can't go any faster (hiring people is a lot of work after all), but in reality they CAN go faster with more people working on the various problems that make up the system.
Of course there is a point of diminishing return. The management costs come into play. The communication inefficiencies. The process issues. Those can be largely solved with good leadership, however.
It's a rule that is only true when it's true, which doesn't make it that useful to me. I've grown teams on multiple occasions from one or two developers to much larger numbers. While productivity may not have increased linearly, it sure made the projects go much faster. Hell I'd argue that doubling a two person team more than doubled productivity just because the external drags tends to be more distributed which reduces the context switching developers are having to do.
I think there are many more shades of grey that inexperienced founders just don't recognize when they're bound to that idea.
