All Human Knowledge

I wonder about the source of the authors estimate of "all human information". It sounds pretty small.

For instance, there is a local broadcasting tower nearby which actually has a Wikipedia page. That's stunning by itself, given it is a boring small tower from the 1970s with nothing special. But I hardly can imagine that Wikipedia will ever see the floor plans of this building. Maybe the city/operator will release them eventually in some kind of open access format, but the Internet is still bigger then Wikipedia.