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.
If you watch the video, they say that the winds mostly blow west-to-east, opposite the Earth's rotation, and they can't counteract that. They show the balloons traveling across 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.
Just cover the continent will require a lot of balloons. The world has 148,429,000 sq km. If you cover 1600 sq km per balloon, that is 92768 balloons. Just cover Africa will require around 20k balloons. It will only work out, if Google can increase the coverage area per balloon.
20K ballons doesn't seem like that many unless each one costs a ton of money. From an industrial production point of view it barely seems like a drop in the bucket given how many cars/phones/planes we produce.
Each balloon would cover a circle, so it'd be more like 1,250 sq km. I think it would be optimistic to expect each balloon to cover exactly that area. They'd need to overlap just to eliminate gaps, and I'm sure there's a random element to their motion, so that would require them to overlap more.
