The extension cords mentioned are all interior-to-interior. There is no magickal difference between an extension-cord wire and a like-gauge Romex wire. The former bends easier.
I've run my share of long extension cords and chained power strips, running every which way, in both rentals and owned dwellings, powering space heaters and everything else. One thing you really need to watch out for is the connections - even a good tight plug+receptacle has higher resistances (electrical+thermal) than the cable itself or a hardwired terminal screw clamp. An older looser-fitting receptacle can make poor contact with higher resistance and even arcing, making a lot of heat. And also the risk of physical damage to the cable of an extension cord is generally higher. So do what you will, but make sure you're inspecting such runs often, especially for excessive temperature rise at the ends.
For any type of generator running exterior to interior would require rated extension cords to reduce fire risk.