> Their slicer is open source but it downloads a plugin once you launch it if you choose to which is closed sourced that interacts with their APIs.

It is very dubious way to subvert GPL, even GPL2, not to mention [A]GPL3.

It was discussed many times that you cannot have close-sourced plugin for GPL host program, as loading plugin is linkage and it is covered by full GPL (only LGPL has linkage exclusion).