This is a very narrow definition of what constitutes racism, one that unfortunately has the broadest acceptance in American culture. By this definition, racism must be deliberate and an aspect of a person's identity. The person must identify as being or not being a certain race, and applying an absolutist belief that one race is superior or inferior to the others. Further more, that valuation is taken as something concrete and consequential, upon which the person must act when race rises as a relevant factor in their social interactions with others. Most people are not racist in this way because it is stupid and stigmatized. But if CSE is not racist or sexist, how do you account for the extreme overrepresentation of white males in CSE?
But that is not the sort of racism (or sexism, which I'm not including for brevity,) that articles like this are addressing. The title, by including the term 'racism' is unfortunately inflammatory because almost nobody wants to be identified as a racist. A better title would have been rephrased as 'unintentionally discriminatory' or something similarly benign instead. Racism, as it is currently understood within academia and sociology (and by most non-white people in America), is the attribution of the assumed qualities of of a group to an individual based on their perceived race. Everybody does this. We visually evaluate people and make snap intuitive judgments, and unfortunately, race factors into this, even if only at a subconscious levels beneath other factors such as socio-economic class and beauty. At our best, we try to mitigate the affects of these judgments in our day-to-day lives, but at our worst, we pretend they are justified. It should not be seen, though, as a matter of the person being a racist person, but rather a particular judgment or action being perceived as racist. If you ever get called racist, you're best off apologizing for the act or judgment and moving on. Denying that something is racist more often falls in line with pretending it is justified, although the intention was more likely to have been to disassociate oneself from the definition of racism in the previous paragraph.
What the article attempts to address is not an exculsively race-related issue, but one that also ties in heavily with class. For dealing with black subsets of the population, there is an unfortunate overrepresentation of black people in the lower socio-economic class. Though I forget the specific statistics, I believe that the approximation is that where black people make up 10% of the American population, they make up 50% of the lower class, meaning that where class-based discriminatory practices exist, they will also be consequentially racially discriminatory (though not inherently racist). CS is heavily class-based in its discrimination, as access to computers and appropriate education is much more limited to families in lower classes than to those coming from more stable or privileged socio-economic backgrounds.
If the article's cultural design tools are meant to address an underrepresentation of non-white minorities in CS, there is value in that, but it is not entirely un-problematic. Things such as teaching cultural histories and linking them to CS reinforce the ideas of cultural/racial identities and could be in that sense considered racist, and the graffiti art could be much more considered classist, but until the groups are no longer disproportionately overrepresented in the lower class, offering cultural and lower class-based points of identification for people in a predominantly white, middle class area of study is one of the only (and statistically most effective) ways to encourage the underrepresented to cross the culture gap. Overall it's not a perfect solution, but it is a corrective one. The only other real options are pretending that racial, class-based, and sexist discrimination are not relevant in CS, and that unfortunately leads to doing nothing and consequently perpetuating the same discriminatory practices that are currently in place.