I have children aged 13, 10 and 5. The oldest spent 1 year in a traditional preschool, but they've gone exclusively to Montessori school since that time.
What strikes me about this article is its characterization of Montessori schooling as largely unstructured and free. I think it must be comparing it to a much over-structured methodology, perhaps like the public schooling I got growing up.
Styles vary somewhat among Montessori schools, but what I've seen is that in the early years, the age Montessori is most known for, there are specific materials children work with and specific ways they're expected to work with them. A child may not get out a work he/she hasn't been shown how to use. He must return the work to its proper place before selecting another one. The materials aren't tools for self-discovery. They're tools for letting self collide with reality until such time as the applicable real concepts are understood.
However, the one simple freedom of being able to choose a work does make it a sharp contrast from the lock-step style of education I grew up with. I hear public schools aren't always this way, according to relatives who sent kids to public school in Lexington, MA.
In higher grades the emphasis on materials fades, but the basic idea of letting children work within a structure remains. For example, in upper elementary (grades 4-6) the students develop their own classroom code of conduct. They're given some structure about how to do it, though. I see Montessori as a balanced methodology on the freedom/structure dimension, not an extreme.