[tesin]

The missing part to the explanation is that Montessori teachers expend a significant amount of effort curating the physical space for learning. The classroom and the materials (with a strong focus on physical objects) constantly allow for the engagement in learning activities. This requires a lot of planning and maintenance. They're also not unattended - the scope is just longer durations. An hour, and afternoon, instead of 20 minutes for a worksheet.

Imagine putting a scientist in an excellent lab, with all the machines their domain required, fully stocked with reagents etc. Then in the morning, asking if three different tasks, which they'd like to do today. At the end of the day, asking what still needed to completed, and adding it to the three tasks for tomorrow. This is obviously a massive simplification, but hopefully it conveys the idea.