Your players don't know you've got a monster who won't pursue. All they know is if they flee and the monster pursues they're dead. And they know the dragon is trying to kill them and they just tried to kill it so they don't have time to suddenly open unlikely negotiations instead of taking a more assertive direct action.This shouldn't be a problem in an actual game - if as a DM I am putting non-level-appropriate encounters to low to mid level characters (before they get ability to get away via items or magic), I'm going to make sure they have a way to get away. A dragon's flight speed means nothing there's a narrow set of caves. A monster might be defending a nest, or have other reason why it doesn't want to give chase. Or maybe a dragon can be bribed with a service.
Don't turn monsters into murderhobos who will all mindlessly pursue and fight to the death unless it make sense - many will only kill when there is a reason, and a retreating foe satisfies their goals. Thew dragon may want to leave them alive simply to spread the word of "Don't go there! It's a dragon's domain".
A monster who tries to kill a party who JUST TRIED TO KILL IT IN ITS HOME is not a "murderhobo" in any sense of the term. First, it's their home so not hobo. Second, it's self defense so not murder. Monsters killing people who tried to kill them is not mindless, it makes perfect sense and mindful.
This is, in my opinion, bad DMing if you're expecting your party to know this sort of stuff about a foe because it's in your head. Unless you're strongly signally these things and you have a lot of trust in your own ability to adequately convey those signals to the people you're playing with, expect the party to not flee because they see it as not a logical course of action in the situation.