Much more likely that there's already a set of HP books in most households, so it's a natural slowdown.
Could be, and that thought crossed my mind too, but the Variety article states:
"The lagging returns for Rowling are especially significant considering that in 2019, her print book sales in June rose right alongside sales for the rest of the industry: 35.2% for the author compared with 33.3% for fiction overall"
So, last year the rise in sales was in line with the rest of the industry, whereas this year it appears (in the US) to be way behind.
In the Guardian article, I don't think Bloomsbury specify whether Rowling's rise is in line with the general rise, only that there is a general rise and that she remains a bestseller. Still, I would be surprised if her trans views were having an impact here. But the US data does seem to suggest them having an impact there, prima facie.