Maybe I phrased it badly, the scenario I have in mind is this:

you have a house, taking power from the grid. Then you want to become more self-sufficient and add solar, but don't want to sell to the grid, just for yourself. If the solar installation and its circuit is not _fully_ separate from the one connected to the grid you have to get a permit even if it's a "Nulleinspeiseanlage".

Yes, technically you could make everything completely separate, then it would be fine. But I haven't seen that in the wild a single time so far.