Did they write "mob"? I don't think the word "mob" applies to UO. Maybe a minor case of "let's take this code from Game A and let's see how it works for UO!"? Trying that could mean one person got to do the work of 3 previous programmers.
Afaik we don't have mobs, we have monsters. And we don't PvE because we are environmentally friendly. We do PvM, i.e. we fight against monsters. Monsters that otherwise might damage our environment.
"Mob" is short for "mobile" which refers to any game object that has a movement characteristic. A "mob" doesn't necessarily need to be a monster - it can be a player, NPC, or a pet as well, but we generally refer to monsters as mobs simply for the fact that the code calls for isMobile() fairly often in reference to checking to see if a game object is a monster (we'll then make more specific checks for players, pets, and NPCs to eliminate down to a monster).