That's completely silly. Yes tail events are important to consider. But major terrorist attack are already events on part of the tail.

All of the successful attacks around the world in the last 15 years probably could have been far more deadly if the terrorists only goal was death count. That's probably not what they're optimizing for, this is framing everything from the victim's point of view. Terrorists can be awful, but they still have motivations and are somewhat rational actors.

I'd be happy to change my view with evidence otherwise, but terrorists are still somewhat "rational actors" with goals that seem horribly irrational to most people. Killing 10 million people in a terrorist attack probably wouldn't serve their long-term goal (even terrorists worry about PR) and would be an incredibly difficult / expensive undertaking.