The horses definitely know. Even if the sitter is totally experienced and competent, she is different, and this can impact herd dynamics in a major way. As a former BM, I’ve seen it plenty- horses acting totally out of character, both to humans and other horses, when their routine gets shaken up or they have new handlers. I once had to leave a barn because the manager didn’t get along with my horse and his behavior rapidly deteriorated (only with her). If I hadn’t seen it, I wouldn’t have believed it, but it came at a time when I was injured and not out to the barn frequently, and she became his primary handler. Typically a bottom-of-the-pecking-order, submissive type, he displayed aggressive and dangerous behavior that has never appeared before or since (owned him for 12 years). It was just a bad fit.
I wouldn’t think too much of the sitter’s snark. Yes, it’s rude and unprofessional, but she was probably very stressed about the situation, and frustrated that all of these problems were occurring out of the blue. I’ve done a fair amount of farm-sitting and it is a lot of pressure, even without issues or injuries. That doesn’t excuse her attitude, of course, but I would guess underneath that, she feels guilty about what happened- even though it doesn’t sound like it was her fault.