> One thing I don't get is what the demographics of the user base has to do with anything? If I buy a Nikon camera or Lenovo computer, should I believe they need to hire more Americans, or even Americans of my ethnicity?
Twitter's product design has much more of an editorial influence on what happens with the product than Lenovo's or Nikon's. A closer analogy might be a local newspaper: I absolutely would feel uncomfortable if the entire staff of my newspaper were outsiders to my community, choosing which articles to run and how to lay them out and how to seek writers, even if the writers themselves were people who lived in my community.
But since you brought up cameras, it is worth noting that the answer is yes. HP had an incident a few years ago where its webcams' face-tracking feature just could not track black people. (Competitors, like Dell, were fine.) If they had a nontrivial number of black people writing and testing these cameras in-house, they would probably have noticed this well before their customers did.
Google, a few weeks ago, had an incident where it literally auto-tagged photos of black people as "Gorillas." Again, presumably what happened is that the dogfood users were much more likely to train the algorithm with pictures of gorillas than with pictures of black people.
Twitter, like most consumer-facing software, has its product design influenced by its own developers. If everyone at the company uses Twitter, it's natural to prioritize features based on what they themselves use. (And it's because of this that consumer-facing software is generally plain better than enterprise software; the developers and PMs and testers care more.) Given how much of Twitter's userbase is people using it for protests, from Tahrir Square to #blacklivesmatter, and given how much "Black Twitter" is a thing, it only makes sense to make sure that at least some of their employees in decision-making positions care personally about these use cases of Twitter.