They really aren't. Recently (2021-2022) Mexico conducted a large random survey of the population and their results were within margin of the oft-claimed 1.7% number (their rate was 1.3% for the sample). The paper linked does some further analysis on those results [1] but the raw data is available at [2].

And their survey evaluates intersex conditions as those present at birth (even if they are discovered later in life but were present at birth).

1. https://doi.org/10.1093/pnasnexus/pgaf126

2. https://www.inegi.org.mx/programas/endiseg/2021/

Wasn't that claim about people who had surgery (or a condition that is visible for which they usually intervened surgically) at birth instead of all intersex people?

I really don't think anyone considers the case of Kathleen to be intersex, seems more like a strawman.