A deer (pl.: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) and Capreolinae (which includes, among others reindeer (caribou), …

So, yes, it is a deer. Because caribou are a type of deer.

It would be like seeing a photo of a pigeon that someone called a “bird”, and saying “that’s not a bird, it’s a pigeon”.