I swam in high school. I wasn't even particularly good, but our varsity team doubled 2x a day 5x a week for 2 hours, plus a practice on Saturday and an hour of dryland. JV doubled 3x a week. That's 19-23 hours per week of training volume. Most kids had been training at that level since they were ~10, some earlier.
Because swimming is a low impact sport, you can train at extremely high volumes from a younger age than running. These days I'm a very recreational marathoner, but even running 70 mpw, I'm not even spending half the amount of time I spent in the pool growing up.
Also, just because it's not high impact doesn't mean this is always great to train like that at a young age. There's a huge level of burnout with the sport. Also it was extremely common for us to have overtraining like symptoms, such as losing our periods (even at way higher BMIs than most endurance sports), getting sick all the time, and never getting enough sleep because you had to wake up every day and get in the pool by 5:30 am after staying up all night doing homework.