While I don't disagree that we don't multi-task - we only task-switch - I think the root of the problem is with the interruption.

Back in my coding days, I tracked the number of interruptions while I was doing code development. I definitely saw the direct relationship between that number and the number of defects injected in my code.

The lesson learned? Close and turn off things that can disturb you - phone, email, door, ...

You will stay in the zone longer, making you more productive and efficacious.