I didn't have any such file in C:\Windows\System32 , so I downloaded this:
https://sourceforge.net/projects/libusb-win32/
Then used the bin/inf_ward.exe to create an inf file and still this driver, while Onyx was in edl mode. As a result "Qualcomm HS-USB QDLoader 9008 (COM5)" in "Ports (COM & LPT)" was replaced by this driver "QUSB_BULK" (that was the default name in the inf_wizard.exe) in the libusb-win32 devices. The file libusb0.dll appeared in System32 as well.
The edl.py stopped complaining about "main - [LIB]: Please first install libusb_win32 driver from Zadig", but it still says "Unknown mode".