More likely that the target depth is down near 50m, which is well into the narcosis range for surface gas mixes. Particularly for inexperienced divers it is very easy to not realize what is happening and become too stupid to save yourself.
Higher salinity may exacerbate that but not by too much.
I haven't been to the blue hole, but I can easily see nonchalant guides leading people to ignore any misgivings they had from what little training they may have had years ago.
I can see someone with a 30m certification going to 40-45m. But anything below that ought to be a huge "no" for anyone who's done the basic deep dive training, in fact you should be running away as fast as you can.
Higher salinity may exacerbate that but not by too much.