I just finished the article, and saw a reference there to a New Zealand study about Rudolf Steiner schools. I attended the same Steiner school as the author of the study, and I would consider several people from my class to be functionally illiterate. This mostly comes down to the quality of the school rather than the philosophy - when I joined it was very new and run by a bunch of well-meaning hippies who had basically no idea about education.

If you're going to have a child-led education system where you assume that if the child isn't reading it's because they're not ready yet, you better be damn sure you're not missing the signs of something more serious, because if you do, when you come to address it it will be much more difficult the later you intervene. It sounds like the teachers in Finland are probably good enough to catch this - ours were not.