I'm not sure why it baffles you, this type of "names-by-associations" are pretty common. See "woodpecker", "ladybug", "daddy longlegs", "silverback" etc... Not

I'm not sure why it baffles you, this type of "names-by-associations" are pretty common. See "woodpecker", "ladybug", "daddy longlegs", "silverback" etc... Not that different. Also maybe "shadow" had a slightly different meaning back then or the translation is simply not 100% accurate (are they ever?).