It's not that unusual, especially in games. For instance,"The Witcher 2" version 3.3 [1], "Dota 2" version 7.32c [2], but also "Struts 2", version 6.0.3 [3]. Some others may be more considered a product name than a version number, e.g. "GNOME 2", "Winamp 5".
I'd agree, but they are calling it "WSL 1.0.0", which is reflected in the release name for example ("Microsoft.WSL_1.0.0.0_x64_ARM64.msixbundle"). Which is natural of course because they are already calling the WSL2 repo simply "WSL".
[1] https://en.cdprojektred.com/download/patch-3-3/ [2] https://www.dota2.com/patches/7.32c [3] https://cwiki.apache.org/confluence/display/WW/Version+Notes...