That leaped out at me as well.
Piaget demonstrated that brain development tends to come in spurts that are closely correlated with growth spurts. One of the key ones generally hits in the range 5-7 and gives a sudden improvement in ability to learn concrete mental operations such as the ones involved in reading, writing and arithmetic.
By contrast the decades long US trend is towards pushing abstract learning younger. There are some statistics indicating that this trend has been strongly correlated with increased gender disparities in school motivation and performance in favor of girls. Which makes sense since boys tend to hit that initial spurt later, and so are more likely to have bad experiences with the start of school. That could easily lead to long-lasting bad attitudes towards school that will guarantee poor performance later on.
Also the Finnish attitude of less school makes perfect sense for me. Because the key is to maintain interest and motivation. The actual hours of instruction we need are not that great, but increasing hours at some point brings diminishing returns on instruction at the cost of frustration and pain for the students.
This brings me to homework for obvious reasons. Research on the benefits of homework show an interesting mixed effect. Homework is practice, but practice can as easily fix wrong ideas as right ones. So homework drives responsibility for teaching to the ability of the home to ensure correct practice. The result is a strengthening of socioeconomic factors on school performance but little net difference in learning. Of course the strongest correlation found is that quantity of homework correlates with conflict at home. With said conflict arising due to the attempts of parents to get kids to do homework they don't want to do.
So summarizing, delaying instruction until kids are ready to learn, giving them sufficient high quality instruction that they do learn, and then limiting the quantity enough to prevent frustration is likely to lead to sustained learning. And sustained learning has good results. Makes perfect sense and makes you wonder why the USA takes the opposite approach on each of those things.