If they reverse engineered the drivers then why do they need a virtual cpu and a Linux kernel to run them. Is this reverse engineering or just installing software in a weird environment?

Speaking of not just gluing stuff together with usb/ip could one make a virtual WebUSB host kernel module that could be used by the Linux kernel USB stack? They most likely would not want to do that because then all of the code would be GPL and would have to be shared with the public.

I don't think a usb host driver is necessarily tainted into being GPL? But if it is, plenty of non-gpl oses that can run SANE.