Maxterm: misinformation or different approach to understanding
Indeed, the ordering of indices doesn't matter in that $$ΠM(1,3,6) = Σm(1,3,6)$$ holds both for the straight and for the reversed orders of indexing, providing these are identical on the left and on the right of the equality sign.
The value of $$f(x_1, x_2, x_3)$$ does depend on the index ordering.