AFAICT they are not trying to provide current browser usage stats. They are only after having a list of popular browsers which is only a subset of the former.
The last paragraph about scraping seems to indicate that.
So they only need what is a reasonable UA as of this week. They don't need what is the most popular one.
I would accept this argument if the sample was unbiased but noisy. In this case it's extremely biased but (potentially) low in noise.
If people from Uganda aren't part of the target audience of this site, we won't get Ugandan user agents even if they happen to be a fair chunk of web users worldwide (certainly more than in my small but high-tech country.)
So they only need what is a reasonable UA as of this week. They don't need what is the most popular one.