Do I understand right, you want that it opens a folder in the same instance in a "tab"?
Maybe then you need "Settings -> configure dolphin -> Startup -> General: -> check -> Open new folders in tabs"
But this only works with
~$: dolphin . & dolphin .. & dolphin <folder> &
you open dolphin with three tabs,
but if you start dolphin like
~$:dolphin & dolphin &
then you have two dolphin instances/windows.
So if you specify a folder to open, it will open a new tab in the running instance, but if you do not specify a folder it will open a new window (like dolphin --new-window), with that setting!