It's not just that they wrote some very optimized code -- though it's true they did. They also designed the app in a way that requires very little server infrastructure compared to other chat apps, especially in the per-acquisition days.

For example, they didn't store contacts/images/messages server-side as you see in Telegram, Google ChatAppOfTheWeek, FB messenger, Twitter, IG, etc. All the infrastructure and the folks required to develop and maintain it, simply didn't exist. Similar with the limited amount of data collection they did at the time -- if you don't log it you there is no reason to have a team of people to analyze it. If you don't have ads you don't need an ad sales team. Etc.