My takeaway from your anecdote is completely different: when exactly a school introduces concepts doesn't matter as much as the stimulation a child can receive outside school.

In my experience, kids whose parents help teach concepts and don't just leave things up to school generally end up with an advantage. Kids love learning from their parents and other loved ones, but school is generally regarded as sort of a chore even if it does bring friends and playtime. A great many parents simply don't have the time or energy left after their day job to support their children the same way others can. Kids whose parents often read to (and with) have a noticeable advantage in many school settings, and you can't substitute that for all students by just cramming in more reading time in their busy school schedules.

Kids have a finite time they spend in school. You can shuffle the time they spend around all you want, but in the end every kid requires a certain amount of time to grasp a certain context. That time may differ when kids get older or younger, but the required time spent on learning won't suddenly change.