The balloons are not going to move in one direction though. The video also mentioned you can steer the balloons, which means, there is way to keep them above the ground and not the ocean.
It doesn't help the way the landmasses are oriented in the southern hemisphere. I think it's a given that the majority of balloons will always be over water. They mentioned a demo in, I think, New Zealand with a handful of balloons. It would be interesting to know how long they'll have for the demo before the balloons are pushed out to sea, and whether the coverage area will be constantly drifting.
The video seemed to show the balloons in a thin band encircling the Earth. Aiming to cover a thin band would seem far likelier to be achievable than spreading them over a broad swath. They also mention using the steering ability to form them in clusters, which is probably what you'd need to solidly cover an area.