Whether it’s more or less effective than interrupted, “zoned” work is going to depend on the individual and the circumstances, but it is certainly a way to work.
It really works - at least for me.
Is it hard? Yes, very, but it’s just a matter of discipline and determination, in my opinion.
I kinda don't believe the people who say they 100% have this job, all the time - but you also don't know beforehand when you need your interrupt-free time. Some days are just cursed and you get nothing done, even if they only interrupted you 3 times, but it was the important 15min block. And I've had those jobs as well. If someone (different people) interrupts you every 15min, when the day is cluttered up with 3 meetings anyway... I think it scales exponentially.