It's a particle verb. In fact this would be the most normal way of phrasing this in Swedish. I know we have some sentences where dogs eat cats or whatever early on in the course with just äter, but in reality that's less idiomatic.
So, äta upp is a particle verb, which means that the particle upp is always stressed. The meaning is sort of that it eats "all of" him. In practice, a sentence like this does not have to mean that the lion ate every last piece of this poor man's body, it's enough that it ate enough to kill the person. So I guess we could say that the particle upp adds the meaning of something final, that the action somehow gives a result.
And in Swedish, this sentence would be a lot less idiomatic without upp. It's still an accepted answer but it sounds a little unnatural on its own. It'd be fine in a context like Ett lejon åt honom till middag 'A lion ate him for dinner' where the action isn't "finished" in the same way. Particle verbs are used a lot to express this kind of thing (lexical aspect) in Swedish, but we don't really teach it in depth in this course. For one thing it's a bit too advanced and for another, it's a topic that's hard to teach via this system, since it's often difficult to capture the nuances of meaning in a one-sentence translation. But you're absolutely right that you have the same phenomenon in English, it's just that it's more important in Swedish and more typical of Swedish grammar.