I get

    Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36

Chromium at least does include "Linux" in its user agent string.

I'll take your word for it, but the source code suggests not on all Linux distributions:

https://github.com/chromium/chromium/blob/2594d9f12f00ecdfbc...

Fair enough, it reports a different string for ChromeOS. But as you can see in the OP, ChromeOS has been listed separately at 1.41%.

Right, exactly what I said in the very first comment:

Nearly 6% [4.45 + 1.41], really. Only misreported due to a certain browser [Chrome/Chromium] not including 'Linux' in its user-agent string.