The first myth is that focus is mostly a matter of willpower. In practice, the people who sustain long stretches of concentrated work have usually just removed more decisions from the moment itself — what to work on, when, and for how long are all settled in advance, so there's nothing left to negotiate with.
The second myth is that longer sessions are always better. There's a point past which output per hour quietly declines, long before it feels that way subjectively. Most people notice the wall only in hindsight, looking at what they actually produced versus how productive the session felt.
The third, and most persistent, is that the obstacle is external — notifications, open-plan offices, a chatty colleague. Removing the obstacle rarely produces the expected gain, because the harder problem was never the interruption. It was the absence of a clear next step to return to.
None of this argues against structure. It argues for spending the structure on the right problem.