The presumption that "women" refers to "white women"..
Referring to the Venn diagram used earlier I think that illustrates the point as regards racism (although not intended!). The region representing black people is some fairly small little circle within the greater area that is supposedly non-black (i.e. read white) people, and the similar circle representing women is within the same outer region (the largest area), which represents, frankly, white men..............Not intended, but that is what leaps out of the diagram for me.
So, whilst the usage is mathematically correct, the real question is why these terms from maths are being used when they are not really needed. They give the statements an illusory air of scientific respectability, whilst actually being used in a way that puts certain groups (used in a non-mathematical sense, but works perfectly well, it is an everyday word) as immediately othered and 'less than'.
Ditto the usage of axes of oppression -- First, as regards sex, creates the impression that sex isn't binary but somehow a continuum. Second, once you add extra axes, you create the impression that sex is just one of many things, and add enough 'other' things as seemingly 'equal-weight' axes, and sexism begins to seem like one (fairly small) issue amongst many others.
If I was trying to explain some concepts to someone who already knew maths, then I might use an analogy to multidimensional axes, or talk group theory or set theory. When someone uses these analogies outside of that, then we can only assume the aim is not to clarify, but to distort in some way.