Well, that answers that.
To clarify a bit on my decision to count this as spoilers: It’d be a whole other matter if the chapter had made it clearer that it was Grue, and I’d just managed to miss that, but it didn’t. As a result, I’ve been theorizing on this. Essentially, the Brutus being the narrator made it so that the story hadn’t actually told me this clearly enough for it to not be a spoiler.
Granted, I’m obviously supposed to be using my reading comprehension to read between the lines (which I did do here – I just read the wrong things), and I do appreciate some corrections when I misinterpret minor things.
Corrections are generally a bit of a gray area, really.