Those usage stats are a fantasy build by nicely asking your browser about your pc's details. But the answer is complete fiction. And one people often intentionally set to display Windows because idiotic corporate-created webpages will refuse to work properly otherwise.
(I haven't touched Windows in many years and still I would end up in those stats as a Windows user (and Chrome which is also wrong)...)
How can the total number of all Desktop Linux users or devices be known to anyone?
The fact of the matter is, none of these stats actually measure the number of users. Most of them are just totally flawed guestimates based on what is often limited web analytics data collected by them.
In fact, not even the developers of a single distribution can guess the number of people/devices using/running that specific distribution. A distribution like Debian for example has mirrors, and mirrors to some mirrors, and maybe even mirrors to some mirrors to some mirrors. So if Debian developers can't possibly know the number of Debian users, do you think OP's site knows the total number of Desktop Linux users?
And let's not get into the fact that the limited data they collect itself is not even reliable. View desktop site on your Android phone's browser. Congratulations! Now you're a desktop Linux user. No special user-agent spoofing add-on needed. You're even running X11. Good choice not following the Wayland fad too soon.
I'm assuming they mean tracking Linux users is difficult since most distributions don't have any kind of telemetry or tracking and there's no company keeping track of their user count like Microsoft or Apple. However, it's not like it's impossible.