From the Introduction to the Language Primary Montessori Album:
We provide the keys to reading, writing and spelling and the children learn spontaneously and joyfully. While we use a phonetic approach, we do not use it in a traditional manner – we use 42 sounds where a traditional phonetics approach uses 150 or more.
So yes, Montessori uses a phonetic approach to reading, writing, and spelling - but that is a huge difference between 150 and 42! Those long series of graded readers? Simply not necessary. Children go from little/no reading to 2nd grade reading level in no time, and within weeks are 5th grade and higher. If they are not, then a crucial key was missed (or too many unnecessaries were thrown in). Or there is another influence within the child (a particular developmental need that should be met another way).