I’m not sure about that inference of “non-distribution”. If the GPL’ed plug-in had first to be installed using an installer, then the installer would be distributed as part of the commercial package.
And even if all the user sees is a notice popping up when they press a button in the app “Acme’s plug-in for greedy commercial software - please read the GPL licence and continue” - that seems rather like distributing the plug-in to me. The FSF representative told me they’d recommend the GPL licence to be distributed in the file package for the commercial app even in that case. But it may depend who exactly replies and their interpretation. It was not a legally binding response.
Gale