Moreover, neither moose nor elk are deer as the caption suggests

Well, that's not true: they're both members of the family cervidae, i.e. deer.

My aunt Kim and uncle Todd might both be related but one would be pretty upset if you called them by the others name

When Americans say 'deer,' they generally mean white-tail or more rarely mule deer; when Europeans say 'deer,' they generally mean red deer. White-tail/mule deer are more closely related to moose than red deer, and elk are more closely related to red deer than white-tail/mule deer.

American from California here, when we say "Deer" we mean "Mule Deer" as the deer everyone interacts with in this state is mule deer.

Being in the same species family does not make it the same. I could call a donkey a horse, yet that would not be true.

Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Artiodactyla Infraorder: Pecora Family: Cervidae Goldfuss, 1820 Type genus: Cervus Linnaeus, 1758 Subfamilies: Capreolinae Cervinae

Really! Us main stream human don't understand this science dialect!

Both Elk & Moose are in the Deer family (Cervidae). Moose are the largest members of the Deer family, & if you're in Europe, Elk are what WE call Moose (Alces alces). Here, Elk are Cervus canadensis, while Moose are Alces alces, two different species. (Deleted prev. post to make a correction)