He never claimed he would find her in an hour.
He said generally they find people within an hour. I even highlighted the important word for you, which would imply sometimes they don't find them.
In saying that, I do think with his expertise in the field of finding missing people, i'd take his word for it. Is it entirely possible that she could be in the water just way further downstream than they have searched. Sure, that is possible, but as said it's often the case that bodies sink pretty quickly and may surface after a few days due to bloating, this river is 15km long, very shallow in places and slow flowing.
Is it possible that a floating corpse managed to float downstream for 15km, without snagging on anything, while going through dozens of twists and turns, without beaching at multiple locations, entering a large lake area, and managing to some how make it through a small opening that leads out to sea where it could potentially just vanish forever.
Sure that could happen, but its so unlikely that the odds would probably be in the millions.