I think that's the same problem as directing it to Night Light - the applet handles things that are in two separate KCMs.
For powerdevil though, there's already an applet that ties with it more strongly, and a button can't really be added to the popup for it like Night Light. Even so, Brightness isn't even really an option to change there, except between battery power scenarios.
Grouping brightness and Night Light as one was the right move, but it produces some interesting caveats like this.