Why does a startup need zero-downtime-deployment? Who cares if your site is down for 5 seconds? (This is how long it takes to restart my Django instance after updates).

Because it increases development speed. It's maybe okay to be down for 5 seconds. But if I screw up, I might be down until I fix it. With zero-downtime deployment, if I screw up, then the old instances are still running and I can take my time to fix it.

If you're doing CD where every push is an automated deploy a small company might easily have a hundred deploys a day.

So you need seamless deployments.