This quote of Dr. Montessori's is actually from a larger passage in her book The Discovery of the Child:
As a precaution, it's important to note that *merely* allowing for such spontaneous experiences wouldn't be helpful for a child, whether at home or in school. Although Dr. Montessori was an ardent advocate for creating and supporting environments in which children enjoy the freedom to be themselves, she was very much against the view that they should be able to do whatever they want, whenever they want. Here's Dr. Montessori's longtime friend and biographer, E.M. Standing, commenting briefly on the topic:
We can allow children the freedom to be themselves while also remembering that they need our thoughtful guidance, structure, and at times redirection. Ultimately, this is freedom within limits.