"Hiring for competence" doesn't automatically get you a diverse labor force, especially if "competence" is measured by metrics that are themselves demographically correlated to begin with (educational background from schools that aren't very diverse, experience at other firms that aren't diverse, etc.). Criteria like that can entrench established demographics, especially if the people who do the hiring look for similar backgrounds and traits that they themselves have... a bunch of WASPs hiring other WASPs because they come from the same schools or like the same governance tools or some framework, for example, or a bunch of Japanese businessmen excluding the white applicant who speaks perfect Japanese because they don't think he'll understand cultural norms, or a BLM group not wanting to hire the white diversity trainer because she ostensibly has worked mostly with Latino populations. There are always unspoken preferences, often cultural things, that bias hiring -- even if you aim for competence -- unless we blind the hiring committee and resumes, etc. A few studies about identical resumes with different last names (suggesting different races) suggest that competence is not really the best indicator of hireability.
> The statistically illiterate HR minions who preach the correlation between diversity and performance don't get this, because they aren't even educated enough to understand correlation/causation fallacy.
That seems kinda like a strawman? Who's arguing that? It's not like having a more diverse team will necessarily result in higher quality code, or more lines of code per month, or some similarly laser-focused metric of "performance".
Diversity can instigate cultural change at a company (do we value hard deadlines more, or should we emphasize work-life balance more?), make it easier to acquire and communicate with customers from diverse backgrounds (cross-cultural communications is hard for anyone, especially so for non-diverse teams with no specific training in it), fill some legal or marketing mandate (we gotta look diverse even if we aren't), help spur innovation in process or product development (what's "out of the box thinking" for one demographic may be a common process for another demographic), etc.
Of course there are costs too. Diverse teams may or may not work together as well, holidays might be different, food and beverage preferences may not line up with current offerings, religious or cultural conflicts may occur, staff polarization becomes more likely (along political/ideological/national origin lines, whatever)...
That's all to say diversity isn't just skin color but cultural backgrounds too, and you get all the pros and cons of that... for better or worse. It's far more nuanced than simply trying to hire for some arbitrary measure of "competence".