Why would you want to have no bootscreen? Take a look at the OpenCore wiki post which shows how to get the bootscreen with OpenCore and you will also see the sectio which tells you how to install Windows 10 for UEFI booting.
I plan to get my Pulse RX 580 8 gig flashed by macVidCards sooner or later but for now I'll have to deal with no bootscreen. At the same time I want Windows 10 on my Mac
OpenCore can give you the boot screen without having to get the RX 580 card flashed. I use an RX 580 and can run Catalina by using OpenCore’s OpenCanopy graphical interface as an alternative to the OpenCore boot picker menu. Also early testing of Big Sur betas using OpenCore suggests OpenCore makes it possible for the MacPro 5,1 use to Big Sur.
The method of installing Windows you are asking about from that website is a nicely formatted write up of a method that was developed by one of the posters on the OpenCore thread, h9826790, as far as I know he now uses OpenCore for booting his operating systems. Why not ask him directly on the OpenCore thread directly what he suggests now. The OpenCore method of booting Windows installs Windows using UEFI. Some Windows 10 installation methods will ruin your NVRAM by overwriting with excessive security certificates which can make your MacPro unbootable.
Aren't there caveats to OpenCore? I've read that some of the USB ports on the cMP stop working
This a an old post now, but still better to let the others know that no such thing exist. May be you messed up some Hackintosh issue with cMP.Aren't there caveats to OpenCore? I've read that some of the USB ports on the cMP stop working