Well, presumably if OP statement were true, recreational diving would involve large numbers of casualties. Casualties in recreational diving are somewhat rare, however, at 2 per million dives.
Tourist dives are even safer since they do not perform advanced dives. Overall, the evidence does not support the view "If you really have only a vague recollection of basic safety checks you shouldn't be diving - seriously."
Most tourists diving have only that and they're not dying in droves.
Of course, because things very rarely go south, especially to the point of someone dying. But still, spending half an hour reviewing some basic course material is not a huge thing to ask.
Tourist dives are even safer since they do not perform advanced dives. Overall, the evidence does not support the view "If you really have only a vague recollection of basic safety checks you shouldn't be diving - seriously."
Most tourists diving have only that and they're not dying in droves.