If they were putting their finger on the scale, the results would have been the same for both.
I hadn't heard anything about this and don't really care if Google is putting their thumb on the scale, but your assumption is incorrect.
If they were using basic semantic algorithms, then the two would have similar results. If they were to intentionally "fix" the result of "American Inventors" it would have a different result than semantically similar phrases, unless they bothered to think of and "fix" all similar phrases.
The search "American Inventors" returns identical results as "Black American Inventors" and "African American Inventors", and almost identical results as "Black Inventors" even without any mention of "American" at all.
That is odd and suggests the reason is not because of the common phrase "African American". Also contradicting that hypothesis is that "American" doesn't trigger "african american" in other searches like "famous American woman" were Oprah and Michelle Obama are the only black women in the first 14.
And "American Inventors" returns completely different results than "North American Inventors" or "United States Inventors" which are both highly similar to each other (as they should be if determined by semantic overlap).
No remotely valid semantic algorithm would treat "American inventors" as almost identical to "Black Inventors" but completely different than "North American Inventors". Something is clearly amiss with their system. Whether that something is random glitch or a done deliberately I'll leave for others to speculate about.