co-founder and CTO of Stream here. I did not write the article but I was very involved with the decision process.

Your assumption is wrong, we did not pick a different language just for fun or because we wanted to learn something new :)

Python was just not the language we needed for our APIs, we tried hard to make it work but eventually we looked at alternatives that would allow us to keep things simple and performant. End result was better latencies, a much simpler architecture (less async/MQ work) and processes that would eat a fraction of RAM and CPU compared to Python.