"Or I can be rigorous and not make data points up in my head to fit some worldview."
It seems clear to me that you're sticking your head in the sand, not me. I'm believing the thing that is dangerous to believe, not you. I believe it because it's obviously true.
"actual hard data" would be best. It would be best if we just had board scores split by race. But we don't. We do, however, have lots of other information that makes it very, very clear that there will be significant disparities by race in USMLE boards in pretty much exactly the same pattern we see in MCAT scores.
Here's the meat of it, you can look to the other comments here for the potatoes:
This: https://www.sciencedirect.com/science/article/abs/pii/S00904... Suggests MEDIAN USMLE step 1 scores for White, Asian, Hispanic/Latino, and Black applicants were 242, 242, 237, and 232. It's urology specific, and practice specific, though.
This: https://onlinelibrary.wiley.com/doi/full/10.1002/hsr2.161 Says The mean (±SD) USMLE step 1 score was significantly greater among non-[Black or Hispanic] applicants as compared to URiM applicants (223.7 ± 19.4 vs 216.1 ± 18.4, P < .01, two-sample t-test). This is at a specific medical school.
...and this is just the result of a casual search.
The people who are best at this sort of thing are economists. They are trained to do causal inference based on patchy, far-from-perfect data. It's totally normal to come to a conclusion (even a very strong one!) using a "patchwork of studies". That's just life. You don't usually get "actual hard data". It's very clear what the pattern in the USMLE data would look like. I bet the effect size would be a little attenuated.
Your epistemic stance, which seems to be "Well we don't have perfect, incontrovertible proof, which means we must act like we don't know anything at all!" is unworkable. You don't do this, I don't do this, the world doesn't permit of this. As a rhetorical move, I can see where you're coming from. It gives you license to not think about the hard thing, and to punish those around you who might. But I'd argue that's not a way forward for us as a whole.