I don’t believe the documentation is a mistake.
With Win7, the QWT installation does provide Windows USB support. (Win7 is so old that Microsoft ended support in January 2020. But when that page was written, it did work.)
I haven’t tested Win8, but it should be like Win7. (Win8 lost mainstream support in 2018, and will be completely end-of-life in 2023.)
When Win10 first came out (2015), it was mostly backwards compatible with Win7/8. However, the 1607 update (2016) changed that and created a split between the installers. This is about the time that cross-compiling installers from Wine to Win10 stopped working. This is about the same time that you start seeing forum postings where people are complaining that Qubes USB support isn’t working in Win10.
When Microsoft ended Win7 support in 2020, people were forced to switch to Win10. This coincides with a large increase in forum complaints that QWT isn’t working for Win10.
In 2020, Wine received a major update in years to fix compatibility with Win10. I don’t know if the QWT developers are using the latest Wine or something older; but it doesn’t really matter: Win10 received a major patch (1903 in 2019) that hardened the application sandbox. While these Wine updates permit Win10 apps to run in Wine, they lack some of the sandbox requirements for making a proper installer for Win10. (Win10 to Wine mostly works. Wine to Win10 mostly fails.)
As far as I can tell, the only problem with the QWT for Win10 code is in the installer scripts. If you run the installer and then look for where it placed the new files, you will see that it failed to install any of the dll and sys files. This is because the Win7 installer no longer works under Win10. If you manually copy the DLL files to the right locations, then they work. The same goes for the Xen tools for Win10; they fail to copy required dll files during the installation.
To reiterate: I really think the problem is in the QWT installer (exe) and not in the DLL, SYS, or qrexec service executables.
I managed to get USB for thumb drives working on Win10 after manually copying dll files. However, I could not get support for generic USB devices working. I’m only manually copying two dll files; I’m certain that I’m missing other required dll and sys files. Unfortunately, there is no documentation that says what files are needed (dll, sys, exe, etc.), where they belong, or what registry changes are required.
If someone could provide the what files / where do they go / what registry changes, then I could complete the manual installation on Win10 and test whether full USB support works.