Four to five hours a day of alternative training such as aqua-jogging does not mean 4-5 hrs/day of aqua jogging.
I would not put it past him to do an hour in the morning and an hour in the evening sometimes, but I'm sure that there was plenty of cycling, maybe xc skiing, roller skiing, and so on.
As to the effectiveness of doing it, I am a big proponent of running in the pool. I did that in college for 45 minutes a day my last two years when I was injured. Although I couldn't even walk up steps with my left leg and couldn't do intervals without my knee stiffening up and not being able to bend it, I ran an 800m pr by 7 seconds from high school in my first race after 8 months of not being able to run much of anything, and the second time out I had a shot at much faster. My speed was substantially greater, in fact, than it had ever been. The point is that you can get in great shape this way. It won't work for all injuries. I have been doing it again recently because of a calf injury and it seems to make my calf cramp up instead of getting better. However, you can do quite high intensity for a long time, a lot longer than on land, with other injuries.
Filip also did aqua jogging in in the Spring of 2017 or 2018 after injuring his foot at the Stanford meet, yet he recovered quickly enough to either medal at world's that year or run 3:30 at Monaco last year.