Article author here, your question is actually a really good one as I'm a bit of an Nginx newb and your comment had me wondering that too. However for the past few hours since this article has been shared the hits have gone quite crazy according to the Raspberry Pis access.log, and the Pagekite.py process is constantly around 30-70% CPU usage, implying a heavy load as normally it uses nearly nothing. Does Nginx reverse proxy cache by default? I'm assuming not given my current results.

no it wont do that by default, there would be quite some wild assumptions to be made to make it a default for everything. even if cache behavior would be signaled from upstream servers properly.

Not by default.