You've not seen someone panic and bolt then when vis goes to zero? On their first or second dive in these conditions it absolutely is a risk of an embolism if they bolt and don't exhale. In our area doing something like this doesn't just result in low vis. There is so much silt in our local spots this can result in no-vis. At one quarry it's like lights out now. If you're not in touch contact or on a line your buddy/student is gone. New diver, suddenly alone on their very first dive? Panic sets in, they bolt, they don't exhale, they embolize, they die.
I'm going to also guess, and I hope they verify yea or nay, that they were being led around single file. Another setup for panic.