It would be better to have a suite of controversial books and check those - I agree. That wouldn't really be a "litmus test" though - that would be more of a "test suite" which is a bit more effort than I care to invest in the claims of random groups that they oppose book banning. A litmus test, by definition, has a singular pass/fail check.
Regarding the popularity of The Turner Diaries - I believe I already addressed this. It has similar search volume, per Google Trends, as one of the example books from the article - "The Black Flamingo" - despite being published ~50 years earlier. So, I don't think popularity is a determining factor in what books the BPL is making available.
Furthermore, effectively banned books, by definition, cannot be popular. If they were popular then the bans on them would not be effective.
I feel that I've let my desire to engage in "point by point" internet-style arguments derail me though. Why the BPL does not have The Turner Diaries is beside my point - maybe they don't like it ideologically, maybe it's just not popular enough, maybe they've never heard of it - who knows.
Instead, the point I want to be making is more like - "If you think book banning is bad, would you want to make this book that advocates for genociding non-white people available to kids?" Because you should test your principles at the limits and not on appealing special cases.
A lefty 2SLQBTGIA+ activist may think and say that "Book banning is bad" when they look at a list of 1,500 books about gay sexuality being banned - but that doesn't mean they are actually against book banning, they might just be against things they like being banned. If you want to know if that lefty person is against book banning, find a book that they would hate and ask them how they feel about that. Similarly, if this article were published by a neo-Nazi institute of religious fundamentalism, I wouldn't be asking them "How about the Turner Diaries?" They would absolutely host that book and complain about it being banned. I'd ask them "How about the 1619 Project?"
As I wrote in my original comment, I am actually for book banning. I think some books are inappropriate for young people and those books should be banned from contexts where young people are likely to encounter them. When someone comes along and says "I'm against book banning!" I feel the need to challenge that because I think book banning is fine and good.