is it completely de-googled though (even when play services are not installed)? Honest question. To me it seems as if that's not one of the main interests of the lineage community.

I do use lineageOS myself and am generally very content with it but I am always a bit concerned when I see that the data usage of OS components (which are thrown together as one "app" in the settings) is more than a few MB. The system must do more than just checking for updates. Unfortunately, you can't prevent system components from accessing the internet if you don't intend rooting your phone.