There are several simple volume label checks left in the game after the Safe Disk patch and this was done intentionally. Restoring functionality was the only point of the patch it was not meant to provide a totally NOCD solution. If the game runs through campaign missions without CD checks for you that means you have a modified GAME.EXE
The SafeDisk patch removes SafeDisk copy protection from the install executable(setup.exe) and the games launcher executable(emperor.exe) Generic volume label CD checks are still present in the primary binary (Game.exe). They are triggered in the primary game binary, not the launcher, when the player makes their final house selection after the intro cutscene to start a campaign mission or when a campaign save is loaded. The official patches only let you reach the main menu without a CD, this was not possible prior.
The volume label checks are generic and are not proper copy protection(that was removed), if you have the safe disk patches installed you can literally get around the volume label checks by burning a completely blank CD and naming it EMPEROR1, or 2 or 3 or 4. That's all the check does is look at the name and device type. A blank CD with the right name will suffice to circumvent those measures. Still, the game will not start the campaign until it sees a disk with the right name. This is why a fan cracked the Game.EXE to remove those pesky remaining checks.
The original purpose of the resource.cfg was to let players move cutscenes to their hard drive to speed up load times, funny enough players were still expected to use the disk to prove ownership of the game. This may seem strange now but back in 2001 it was a fairly normal arrangement.
Editing the resource.cfg will allow cutscenes to load of the HDD but the game will sadly still ask you to insert a CD because The CD prompt is not triggered by the game being unable to find files, it is triggered independently. This is why the resource modification is a needed step in a successful noCD solution but by itself will not work using only official patches.
Copies of the game floating around the internet often have the Game.exe cracked already so that one only needs to apply the newer Win10 fixes/patches and they are ready to go, if you are working from original materials..IE real disks, you will not have success getting the campaign to work without CD's even after all the official patches are applied unless you hunt down the modified GAME.EXE
The very forum referenced at the top of the article has tons of posts spread out over the years of people asking why they can't get the NOCD to work after following this guide and the members of the board have to explain time and time again that it is partially incorrect.