Currently, only the applet can set the temperature. The gnome settings option will only configure the time that nightlight activates
The nighlight has a button to open the GNOME Settings Displays option where you change the times.
I personally don’t see the value here to pull in the same functionality directly into the applet - doubtful many people will be actively changing the time all the time.
Anyway - feel free to raise this with upstream https://github.com/solus-project/budgie-desktop/issues/new - but I expect they will similarly need to be convinced as well.