If you do any reading on race and diversity, you'll come across the idea of racism as a systemic thing -- racism not as single instances of discrimination based, but a recurring pattern. This is why it's very hard to insult a white person with a racial slur. It's why 'girls who code' groups are common, but male-only versions feel pretty gross. It's why, in the context of our industry, a predominantly black dev team is really unusally diverse.
(It's also a bit weird that people keep calling that example 'non-diverse', when entirely-white groups of developers are still the norm. Even a predominantly white group -- but with some people of colour -- is unusually diverse in this field, sadly.)
People are calling that example 'non-diverse' because they are using the dictionary definition of diversity: "the state of being diverse; variety" or "an instance of being composed of differing elements or qualities".
By this definition, a team of white, south asian and east asian people is diverse. A team of black Americans is not.
You seem to be using some alternate definition - I'm guessing diversity for you means "sufficient numbers of non-Asian minorities", but feel free to correct me if I'm wrong.
Let's take the immediately preceding example, where the author had a black woman as a supervisor. The author describes this as 'diversity lightning'. If you interpret this narrowly, this makes absolutely no sense: there's no way in which a single person can be considered diverse. On the other hand, if you think about it at an environmental or company level, things look different.
Suppose we take a random US tech company's employees and dump them in a vase. (Let's ignore the ethical issues with this for a moment.) At this point, we'd expect the odds of a random draw to reflect the tech industry's demographics, which is to say, predominantly white and asian males. If you draw this 'predominantly black' development team from the vase, though, you need to update your priors. You probably shouldn't calculate your posterior as 'everyone at this company is black' -- but you'd still probably guess that you're looking at a tech company where there are more black people and women than average. Which is to say, you'd expect that the company as a whole was more diverse.
I think this second reading is a lot more charitable to the author. I also think it's the more interesting reading; the demographic representation in a small group is subject to a huge amount of randomness, but the demographics of a entire organization are more telling, and can reveal more interesting things about policy and bias. As a community, software people are normally biased toward systems-level thinking; it's weird to me that in this particular issue people have this laser-focus on small groups and tokenism.
That is a more interesting interpretation. I think it's more of a steel man [1] than a charitable interpretation, but that's absolutely worth bringing up.
(It's also a bit weird that people keep calling that example 'non-diverse', when entirely-white groups of developers are still the norm. Even a predominantly white group -- but with some people of colour -- is unusually diverse in this field, sadly.)