I have installed all the dlc via offline installers. Why does the game explicitly tell me that by not logging in, I lose access to all my paid content?

(Posting this just for clarity since the OP already has been informed as well.)

This has been proven to be a false positive. The game shows the message that it "may" be possible that not all content is unlocked but that is not the case as you can see here.

https://www.gog.com/forum/general/revenge_of_the_savage_planet_cosmic_hoarder_edition_drm_problem/post9