It peaked at 4.05% in March. The last 2 months it went just below 4% as the Unknown category increased. For June the reverse happened, so 4.04% seems to be the real current share of Linux on Desktop as desktop clients were read properly/werent spoofed.
Connect the Steam Deck to a compatible dock and you can quite easily use it as a desktop. At the end of the day, it's still an x64 based PC that's just handheld.