I don't think any would argue that python is the fastest language. The more pertinent/practical point for most though is not to needlessly overbuild whether it be in dev time or servers.

Realistically, the author could probably serve 10x-100x more users (your million users) and still stay on the cheapest droplet with python. As you can't go smaller than the smallest, changing language can't be justified on a cost basis.

Finally, the performance differences between a well written python app and a poorly written c# app are probably not that large. The developer probably started with python, because that's what they know, and wouldn't be able to match the code quality in c# without significant effort.

Use whichever language you're most comfortable with. You can always add more hardware to the problem; and if you get really big then hire people to start re-writing parts of it.