So I am not as keyed into the interwar British eugenicist literature, but in the interwar American eugenicist literature (e.g. Lothrop Stoddard, The Rising Tide of Color) there was an argument (one that still persists in some of these kinds of circles today) that "Asians" writ large were in fact more intelligent than Europeans. This was usually followed with a caveat that they lacked individuality, creativity, etc. (always some fundamental thing "missing"), but with the warning that if the Europeans ever got complacent they would be overrun, out-produced, etc. (which goes with the alarmist eugenicist creed just fine). Which is just to say, if you are wondering, would this convince eugenicists that they were wrong? For a die-hard eugenicist, not at all: they would of course just selectively interpret this to fit into a worldview they already had.

The intellectual historian David Hollinger has a nice term for this kind of thing: the "booster-bigot trap," where exceptionalisms can be applied in either direction, sometimes simultaneously. (Hollinger uses this to talk about anti-Semitism — which holds that the Jews are somehow inferior but also great with numbers and money — but it works here, too.)

On Stoddard, see esp. Matthew Press Gutterl, The Color of Race in America. Stoddard more than anyone in the US was responsible for the interwar movement away from the "divide up the whites" racial theorizing (of people like Madison Grant) and towards a "whites versus colored peoples" sort of scientific racism that has persisted for quite some time.