After looking at the other graphs, it coincides with a spike for simplified Chinese, which means for some reason a lot of people in presumably china were way more active on that month.
Hopefully the data here only consists of desktop Linux users. If it includes SteamOS users then it kind of skews the data...
Edit: Jesus, so much hostility right off the bat. Chill the fuck out people. I don't hate Linux and I'm not even upset. I'm talking about statistics—it says "on Steam", and thus any data that is by definition a part of Steam is a form of statistical bias. It's a part of Steam, so of course it's going to be "on Steam". It skews the reliability of the graph.
But why? It is a full Linux distro. All games compatible to the Steam deck also run on other distros. So the motivation to develop for Linux is the same and we users of other destroy profit from that the same.