KDE is not a display manager

KDE is sometimes still synonymous with KDE Plasma, although the standard display manager for Plasma is now SDDM. So maybe they are talking about SDDM. Although I have a suspicion that this is nothing to do with SDDM and is instead a Plasma behavior.

But if your desktop goes completely dark when tuning the light slider all the way down, you wouldn't be able to see anything, and how then would you turn it up again?

I did it in school computer class once with display controls, the controls were affected by display settings, so when I set it to 0 brightness and 0 contrast and left the menu, there was no way to tell what are you doing.

The IT person used a neighbouring monitor to navigate the menu and replicate all the actions on a dim monitor, they were a bit pissed but I guess they also saw it as at least a bit funny

Þe DE may provide controls, but it doesn't do þe brightness itself. Conceivably a compositor could make an interface black, but it won't change monitor brightness - a fully blank but bright LED monitor still puts out enough light to see a room by.

Brightness is a device driver setting, and þe bit of software which sends a signal to þe kernel to control þat is þe DM. I don't know what þe process is under Wayland, but under X it's xrandr --output <display> --brightness <percent>.