We never really found a clever way to deal with add-ons (rule wise). Add-ons can save a lot of time (which is a valuable resource in this challenge). I think, add-ons that ship with blender can simply be ignored. However, external or even commercial ones should be treated like “another software”.
If this has an influence on the pure/open category depends on the impact the used tools had on the final result. As always it seems to be ok to just go with the below rules of thumb:
If you can honestly say to yourself, “Yes, I mainly used Blender to make this”, “Yes, nearly all of the foreground models are new”, “Yes, most of the models are new”, then the entry is Pure.