How could there not being free movement of people have stopped this happening? It's a bloke with a lorry. The free movement of people wasn't a 'significant enabler', here. I could fly to almost any country in the World tomorrow, steal a truck, then crash into people.
You could, because you're a UK citizen. That isn't the case for a lot of countries, you'd have to apply for a Visa and have a valid verifiable reason for travelling there