We decided to watch a "documentary" about "legendary creatures" because they seemed like contradictory terms to us. What are these "legendary creatures"? Bigfoot, Nessie…the creature from the Black Lagoon, chupacabras, vampire bats? What exactly?
And the answer is—wilderness wildlife—wild boars, wolves, brown bears, fire salamanders, etc.
I wouldn't consider those "legendary creatures" (except the fire salamander perhaps). That lead-in seems misleading to me. But it's not just that; the narrator is almost whispering, like it's a bedtime story, and uses phrases such as "creatures of folklore" while relaying only factual information about wildlife in their natural habitat (i.e., wolves, not werewolves or any Big Bad Wolf somewhere).
I don't know what you'd call this, but I don't think it's mysticizing (e.g., interpreting earthly events as alien) or mythicizing something or someone (e.g., a historical figure such as William Wallace). It's like the opposite of a mockumentary… I don't know; maybe it's just selling something.
I suppose it's a theatrical, albeit factual, storytelling technique referred to as _____, an attempt to turn a factual story into a compelling tale by cloaking it in nonexistent mystery.
Example of usage I found while spell checking chupacabras (Wikipedia):
The chupacabra or chupacabras is a legendary creature in the folklore of parts of the Americas, with its first purported sightings reported in Puerto Rico in 1995. The name comes from the animal's reported vampirism—the chupacabra is said to attack and drink the blood of livestock, including goats.
As quoted from the search page for simplicity. Emphasis mine (underlining due to added link), but it links to the same wiki page as the chupacabra page does.
Instead of being loosely or broadly defined, I think it's more a matter of whether you define the words together (as in the "legendary creatures" wiki page) or as two distinct words. There is probably a term for that; I may have seen it here before.