An installed battery will have its terminals protected and connected to the device.

An uninstalled battery will have its terminals exposed, where they could be accidentally shorted by a stray paperclip, foil wrapper, or zip.

Danger probably primarily varies with capacity, production quality etc., not form factor (disassembled or assembled into a device), sure.

I'm not saying that current regulations of lithium batteries make sense; my argument is that the actual threat from lithium batteries seems larger than that of devices not in airplane mode (i.e. somewhere around zero).