As a manager in an open plan office, I will say this is categorically untrue. I have twelve people interupting me constantly, and I need to stay late or start early regularly so that I can get some uninterrupted work time. It also tends to work against self sufficiency. Rather than learn how to do something (or find out how to find out), it's too easy to ask someone (like me).
I do have a lot of meetings, but I also have work. I need to read specs, for example, and understand all the implications. I need to plan projects with complicated circular dependencies. These are "load lots of stuff into memory" jobs. It's impossible to do in 20 minutes stretches.
I'm also a sprint-and-rest type of personality. I get a lot done I. My most productive stretch of the week. Knocking me out of a productive roll can be real costly in time.
It also makes me a worse/lazier manager. Rather than make sure people are working on a nice big chunk of something, and have everything they need to get the job done, I can trickle information in as we go. That's invariably worse, but less work upfront.
There are definitely upsides too. You get a lot of cohesion for free, especially at a company in the 50-250 people range.