But everyone is different. Our kids learned well by being allowed to pick what they studied. That may not be suitable for all children, and some, perhaps even many or most, might need more structure and rigor earlier.
All of the messaging around play-based learning was in the negative — "no, we don't do that; it's play based!". They never said "we do play based learning, which means that the teachers do x, y, and z to facilitate outcomes 1, 2, and 3."
Of course teachers and parents played up the projects the teachers worked on, and encouraged learning through play (e.g. understanding why the sand fell the way it did or how the swings worked, etc.). But it was very indirect and light-touch.
What we found was most kids naturally gravitated to things out of curiosity. Most knew the alphabet, days of the week, numbers and the like by the time they went to kindergarten. Many could even read and do simple arithmetic.