One person can keep a rough mental model of how the project fits together in their head, which lets them have good intuition about contracts between different subsystems.

When you have 5 developers on one project, that intuition quickly breaks due to developers mental models being slightly different. That's where having a type-system really helps.

I'd argue that the you lucked out in that the stack was able to be developed by one person, and that's what enabled it to be dynamically typed with relatively little loss.

It's once you have 3+ coders working on the same code-base that static types really start helping.