Those plugins are not plugins: they’re standalone programs called from another program with a file parameter pointing to a disk file. They work on that file with their own tools. When they’re saved in a certain parametric way, those parameters refer only to their tools so they can only be used within that certain program.
In this case every program creates a file existing out of 2 images, the starting file and the result file, and a parameter list. The undo/redo is only possible within that block. I’m not sure if I can see an usability for that.
See the Viewpoint tool. Within PL it acts like any function in PL. When used as a Nik tool it creates an own block. What is the use of that? Except for the sale to non PL owners.
George