Your anger seems, to me, to be coming from the idea that if people assigned male can have endometriosis, then somehow the immense medical misogyny experienced by people assigned female who have endo doesn't exist or doesn't matter and you aren't allowed to talk about it at all. Which just is not true. All of the points you've made about the medical misogyny targeted at those with endometriosis and vaginas/uteri can be true and important, and people assigned male who aren't intersex can have endometriosis.
Plenty of cis women act the exact same way about trans women having hormonal menstrual cycles, insisting that its impossible and everyone who claims they do is lying or mistaking IBS symptoms, and that its stolen valor to claim that its possible. And yet it is possible. The body does not care about maintaining very clear distinctions between sexes for the sake of making political points more black-and-white. Men in pink collar jobs earn less than men in blue collar jobs. That doesn't mean that the devaluing of pink collar jobs isn't misogynistic, or that people seen as women aren't primarily being affected.
I did read one of the studies listed in that article, which is both a case report and a literature review. They surgically removed a mass from a man's abdomen and that is how they diagnosed it as endometriosis. That seems... pretty credible to me?
Gross exam of the mass in the operating room revealed a central cystic cavity with cloudy brown fluid. Further, the results from immunological stains performed by pathology revealed a highly unexpected diagnosis with an immunoprofile consistent with endometriosis. The H&E stain (Figure 2) displayed a layer of endometrial epithelium with underlying stroma. The cells lining the cystic mass stained strongly positive for CK7 (Figure 3(c)) and estrogen receptors (Figure 3(a)). CD10 stains were positive (Figure 3(e)) and CD15 staining was focally positive (Figure 3(d)) in the underlying stromal-like tissue. GATA-3 stain was negative. The patient was discharged on postoperative day two. In a two-week followup appointment, he had complete resolution of abdominal pain. [...]
A comprehensive review of risk factors, location, immunohistochemistry, and outcome of prior documented cases of endometriosis in males has been accomplished for comparison (Table 1). Most of the cases involve increased estrogen in men with liver cirrhosis [2, 6] or prostate cancer treated with long-term estrogen therapy [1, 3, 5, 7–9, 11, 13, 15]. Although this patient did not have the aforementioned risk factors, it is possible that his obesity with a BMI of 35.7 caused increased systemic estrogen levels. In the case reported by Zamecnik and Hostakova, the only identifiable risk factor was obesity as well [14]. Several studies have identified a clear, positive association between increased obesity in men and increased estrogen production [17]. This phenomenon is likely associated with increased aromatization activity of adipose tissue, overexpression of proinflammatory cytokines, insulin resistance, and hyperactivation of insulin-like growth factor pathways [17]. In relation to male endometriosis, it could be theorized that this increase in aromatization could provide pathologically elevated estrogen levels to drive the growth of endometriosis from remnant embryological cells in a male.
& Its notable to me that they use such definite language, because PCOS/PMOS is also believed to occur in people assigned male without uteri/ovaries, but from what I've seen that hasn't been treated by the medical community as a definite reality (i.e that article says "PCOS might affect men" even while saying brothers of women with PMOS have symptoms suggestive of it). So even if there are only 20 cases, that these are treated as sure cases of endometriosis is telling to me. And why would they lie or overemphasize certainty about this, when there's no gain I can imagine by doing so?
Once again, everything you've said about the profound impact of misogyny on people with endometriosis can be true and important and real, and perisex men can have it. These do not contradict each in other in any way, a person assigned male claiming to have endometriosis does not mean you are not allowed to talk about any of that or that it doesn't genuinely affect people or that it isn't misogynistic or anything at all.
If your advocacy for people with endometriosis, people who menstruate, people with uteri, is so easily hampered by the reality that sometimes things which impact us also impact people with penises and testicles and no uteri or vagina, I think that's a weak spot for you in your ability to advocate on these issues. I see no reason why this being a reality should impact your ability to talk about these things in any way. If someone claimed that they are a perisex person assigned male with endometriosis and so the misogyny doesn't matter, or that because people assigned male can have endometriosis it means you aren't allowed to talk about the things you've discussed above, they are wrong.
If it is true that perisex people assigned male can get PMOS or endometriosis, it changes nothing about how medical misogyny shapes the lives of those seen as women with PMOS or endometriosis, and the importance of talking about that.