These things are considered and any decent instructor won't throw an untrained baby into the water. Newborns are first slowly conditioned to be in and underwater. Often you add a verbal cue before submerging them, so the baby starts to expect it when hearing the cue. The throwing happens once they are used to being underwater. And then the throwing plays a big role because most infants who accidentally fall into water will do so from well a fall. By being used to this, you reduce their risk of drowning.

Reflexes generally don't develop as we grow older. On the contrary, we lose a lot as we grow up. For example, newborns have a diving reflex up to around 6 months. So a baby is on average safer underwater at age 0-6 months. Not all infants have this reflex though, so you always need to be extra careful the first few times and should always start slow.

Are there some people who throw untrained babies into the water? Yes, unfortunately, they exist. And yes, some newborns will die because of it. But generally, people who plan on teaching their newborns how to swim start the proper way. So the risk isn't any higher than with a 4-year-old (which used to be the recommended age). And even if we look at 4-year-olds, the methods don't change much. Most people start slow and do it properly. A few bad apples throw their child into the water. And then some children drown because of it.