Nir Eyal explains that timeboxing isn't about finishing tasks in a set time. It's about learning how long things actually take by working without distraction, then using that data to estimate remaining work across future timeboxes.
Chapters
Most people think that the whole point is to finish the task, but that's not what timeboxing is about. Timeboxing, rather, is about figuring out how long things take you to do by working on them without distraction. This is a really, really important point. A lot of times when people start working on a task, they think, well, I didn't finish, so I have to bleed into the next time box. Big, big mistake. Rather, what you want to do is say, okay, here's how far I got in this task. How many more of these time boxes will I need to allocate tomorrow, the next day, the next week in order to finish the entire task? That's the purpose of timebo, to finish the task. Because that is the only way that you will create this feedback loop to inform how long things actually take.