It too confused me at first since the filter kind of sounds like it’s editing the active toolbar but in the documentation of the hook it says:
Once a new toolbar is added to the list, it will then appear in the WYSIWYG field’s options when editing the field group.
So I added the code in your question and sure enough if I go back to edit the Field Group in Custom Fields there’s a new radio button on the WYSIWYG Field labeled ‘Very Simple’ and when selected will only display a toolbar with bold, italic, and underline buttons on it.
