This is not quite what happens. You get attacked by enemies with the threatening note before Elysana invites you. Then when you talk to her, those messages trigger immediately if you have already killed that number of enemies. I would not be surprised if this is vanilla behaviour, but the enemies killed counter should really only start once you have spoken to Elysana and accepted the escort quest.After you have killed a couple ambushers, Elysana's cousin will cryptically interject:
(Cousin's name) says "Who were those barbarians? They almost killed you! Maybe you should just forget about Lysandus' ghost."
If you have slain five attackers and are not in fast-travel mode, the cousin flees at sundown:
(Cousin's name) says "You're much tougher than Elysana said you would be."
Edit:
I see it listed under bugs:
"The barbarians and knights begin attacking as soon as you have received the initial threatening letter. However, you still will receive the cousin's dialogue after killing the ambushers even if you haven't yet met with Elysana."
Maybe something for DFU to fix at some point?