I'm guessing this is not a measure of data at rest, but data transferred over the network. The couple samples listed on the page ranged from 2.5% improvement to 20.3% (vs. zLib) so I guess they're extrapolating that out to all app downloads and updates across the world. Nicely done.
More generally, we've seen some great advances in compression lately. I've been using Facebook's zStandard [1] for compression in a product I'm currently working on, and I've been extremely pleased with both its speed and compression ratio. The days of "just use zLib" are coming to a close.