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.