Choosing your break isn't an interruption. An unexpected, external tangent is.

One single "quick question" doesn't do a lot of harm...

If you have an average of 4/day or more, it will probably consume all of your working time. But one of them isn't a problem at all.

And there's the problem, because it's both not a problem in small numbers, and a DoS attack still in small numbers.

A quick question from a fellow engineer is almost always quick; unfortunately, that's not the case for most other situations.

A "quick question" from non-engineers (and even fresh engineers not yet used to the idea of focusing on work at work, or in general people who don't understand they're not the centre of the world) is usually carrying within it a request (or demand) to do something for the other person. A genuine "quick question" is one to which a quick "I don't know" or "not now!" is an acceptable answer. Those are easy to navigate - if you feel like considering it fully will break your focus, you can just decline answering it. If you can't, it means the question is really a disguised attempt to offload work onto you; engaging with it is giving permission to the other party to pull you in and try to keep you there.

Well, one question probably causes around 10-15 minutes of harm (not counting the time to actually answer the question).

So yeah - have 4 of these a day and you've wasted an hour. Have this every working day and you've already wasted half a day per week.

This seems like a failure of emotional self-control...