Imean, none of that makes it an apex predator. They just make thr hippo a fierce animal. If you want to argue "Steve Irwin was afraid of it" as a criteria for apex predator, you're certainly free to lobby the general scientific community for that.
Which is all just a diversion from the fact that "no natural predators" doesn't necessitate "apex predator". You're creating your own dream ultimate life form. Why make it a predator if you don't need it to be? Again, manatees.