Because it increased by over 3%?

Part of the jump at least appears to be explained by Valve correcting again the Steam China numbers. Month over month they report a 31.85% drop to the Simplified Chinese language use and English use increasing by 16.82% to 39.09%. Other languages also showed gains amid the massive decline in Simplified Chinese use.

So it wasn't a major jump in usage, just a re-weighting of accounts by real active usage.

That's still much better than several years ago in the pre-Steam-Deck days when Steam on Linux was at around 1%.

And a heavy amount of Linux use is just Steam Deck console adoption. So, not nearly as much a cut-over from Windows to Linux as a cutover from XBox/PS5 to Steam Deck.