This has very little to do with terrorism. I'm sure you can find the odd counter-example, but most terrorists enter their target country perfectly legally and are often radicalised while resident in the West. Terrorist organizations know that such people can operate much more easily and are less likely to be monitored by authorities so that's where they focus their recruitment and operational activities. Anyone conflating illegal immigration with terrorism is snowing you in order to push an agenda.
Getting on a plane in EU with EU passport is easy. Getting off the plane in London will get you to a proper security check. They would have caught him there, hadn't he declared his syrian nationality.
Hell, most of the times I've flown between European countries, nobody even looked at my ID. If somebody did, it was usually check-in staff, so there would have been nothing preventing me from checking in under my own name and then handing my ticket to somebody else.