Haven't read the fluff piece myself, but it sounds like appropriate stuff that makes the 40k world as grimdark as it is. I can understand if people that really like battle sisters might feel some aversion to reading about a group of them being butchered by their own side for no reason, but since it achieves a result of showing how inter-imperium fighting can be entirely fluff-justified and that the setting is over-the-top dark, which it is supposed to be as I see it, nothing in this scenario in principle qualifies it as "bad writing" as was originally stated.
Since this thread is very long and I only read the first page, several people have probably said this already though!