Reverse engineering approximations of the other parent’s finances can be done by using the college’s net price calculator; access to the other parent’s part of the CSS Profile is not needed.
Of course, sometimes it is too obvious. E.g. if one parent is poor enough that the student would get full financial aid if just that parent’s income and/or wealth is used, but the student actually gets no financial aid after both parents put in their parts of the CSS Profile, then it is not hard for the poor parent to see that the other parent has lots of income and/or wealth.