Yes, but a python app can be efficient enough to serve that many users on the smallest size droplet. Any additional speed gains would only be useful for scaling and MAYBE latency.
But scaling it to a million users, say. A python app would consume way more resources than an app in C#
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.