I see what you mean.
My son has been in a Montessori preschool/kindergarten for 2 years. It is 100% Montessori, private school. He does very little reading and writing there; we have worked with him at home to get him reading, which he is doing well at.
However, I do think his Montessori is not very good at verbal things, getting kids to read, etc. At least half of the kindergartners finish that year unable to read. He did great on the nonverbal part of the test and good but not great on the verbal. I'm wondering if that is related to the school background.