The email was neither rude nor dismissive. And what makes you so sure he was on vacation? He said he had “returned to work....after being away on leave”. Did it occur to you it may have been a emergency medical leave? Or a bereavement leave? Or leave to care for a sick child or spouse or parent? If that was the case, would you then “be okay with that”? Not that it is really any of your business why he was on leave.