I am from Europe so pardon me if I totally missed the point but how is 90% african american considered 'diverse' ?
The demographics here are pretty stark: the percentage of women, and the percentages of hispanic and black people in programming and other tech related jobs (and studying it at the high school/college level) are way, way below their representation in the overall population.
So "diverse" can both being used to (as another poster mentioned) refer to "non-white", as in the states it's often used as a byword for that, but I think in this case it's being used as a comparison to the expected demographics.
As a much more off topic explanation of why "diverse" gets that use in the US dialogue: the US has serious problems talking about the fact that racism and inequalities along race lines both are still incredibly prevalent; many people would prefer to think it ended after the civil rights movement in the '60s (and that is largely how it is taught in the public school system). Saying something along the lines of "I was impressed by the number of black people/African Americans who attended" or even "I was impressed by how many non-white people attended" isn't politically correct in some circles, (and can open you up to accusations of racism) despite the fact that it mainly acts to highlight underlying inequalities and race lines rather than actually actively perpetuating and enforcing them. However, "diverse" is used as a byword to mean the same thing without actively highlighting any particular racial inequalities, which is more palpable for some people. (In my opinion, more palpable mainly for the people who are uncomfortable acknowledging the problems or who would rather not address them, but that's more social commentary than an explanation of the use.)
In western Europe, Belgium, we would use 'diverse' when there are many nationalities among the attendees (ie: Italians, Moroccans, Turks, African, etc.[0]). Note that I purposely gave southern nationalities to highlight how 'diverse' is also a byword for us: we wouldn't use the word if the attendees were Norwegians, British or German (we would use 'European').
[0] Although nowadays all those people are sadly put under the broader and less subtle 'Arab' or 'Muslim' label.
"Diversity" as a slogan is somewhat misleading, because what these initiatives try to do is to counterbalance negative selection of characteristics like skin color, parental income etc which shouldn't matter in coding.
Also "minorities" has become a difficult term because of spatial segregation, demographics, blending, and even the decline of racial prejudices (which is a good thing) blurs the lines of who's the majority and who's the minority in a particular issue.