Earlier quoted context omitted.
Speech may be much looser, but a classifier to (try to) detect absolute gibberish from real content doesn't look too far fetched to me. Its only action would be to disable the subtitles by default if gibberish was detected. It may be computationally impractical though.
The problem with that is that the language model's power is already used to fix things up locally (because this is transcribed from audio). As a result it can't be used again to decide if the transcription fits the model; it's the case by design. There must be some kind of confidence metric at the end of the process, but I don't think it's possible to tell how much of the ambiguity comes from inadequacies in the phon…
So good transcriptions would be good, regardless of the noise environment. It might give some false positives, but I expect that a good price to pay to avoid the kind of mess they create now.