Even Ashkenazi Jews, who have comparatively pale skin, have considerable genetic heritage from the Middle East: https://en.wikipedia.org/wiki/Genetic_studies_on_Jews#2017%E2%80%93present

Therefore, it is not true that "biologically they were all the same" any more than it is true of those who we currently would classify as mixed-race people. (As humans, we're all pretty much the same.)

Furthermore, given racism associated with the one-drop rule (https://en.wikipedia.org/wiki/One-drop_rule), being hard to tell apart isn't a prerequisite for racism. Jews can be somewhat difficult to tell apart from non-Jews in Europe (even if many of them have some somewhat distinctive features, it is not enough for great confidence). But the one-drop rule was a rule specifically to extend racist actions into cases where it's visually difficult to tell.

Finally, the Nazis did not use the same racial divisions (which are for the most part socially constructed and hence both subject to change and also not normative in any particular sense) as people in the U.S. do today; the "Jewish race" was assuredly NOT the "Aryan race", to them, and that is what mattered to them. The same attitude of racial purity was used to justify in their eyes both their actions against Jews and against, for instance, blacks (https://www.hmd.org.uk/learn-about-the-holocaust-and-genocides/nazi-persecution/black-people/).

So there isn't, as I can see it, any thoughtful way to say that the Holocaust wasn't "about race". It was so about race that it boggles the mind from a modern perspective.

Hopefully in another century people will look back at our time and be equally boggled by the importance we ascribe to race.

(Finally, as a point of clarity: although Hitler was dead, many of his top officials were captured and their IQs were tested: https://historyofyesterday.com/the-results-of-the-nazi-iq-tests-c3a5e442f37c. The terrifying thing is that they were not stupid. They tended to range from bright to brilliant. We mustn't allow ourselves to think that only stupidity can lead to cruelty, or we will fail to guard against wickedness presented with intellectual acumen.)