Except.
It's worse than that. In the Cap'n Crunch whistle case, it's at least a closed problem as to how to filter out instructions from an untrusted stream. You still need to worry about when it's safe to let instructions through and when it isn't, so out-of-band is still much better, but at least you know what frequencies to filter out and the like.
For LLMs, any sequence of words could be bad for your product! Filtering out instructions is a completely open-ended problem!