For the second one (what weakened the authors conclusion) I said it was she wasn't representative of Chinese-American writings. I stayed away from the one about the book because the book was just as example of a larger point whereas I thought the point he was trying to make was that she had a literary lineage if you will. If she's not representative then she may not have that lineage. I'm probably wrong though.
I actually chose the opposite as you, though I did narrow it down to those two. To me, it seemed as if the book not being representative of her work undermines the author's claim that her writing as a whole has a base in the whatevertalk tradition. That book is the only example he gives of her writing, so if that's nothing like her other writing, then she probably isn't heavily influenced by the whatevertalk roots.