Let me know if you have questions (I'm the lucky indie dev who wrote the post)
My wife is a Montessori teacher and would flip her shit if a parent ever substituted the real montessori numbers/letters (usually sand paper letters) for an app. Montessori would've surely been incredibly against an app for children young enough to learn anything from sand paper letters.
Your apps, if nothing else, are an excellent way to distract our kid if we go out for dinner :)