Its a good question... in the article it states that Guifi was able to get a "peering" connection.

Internet service providers have to connect to each other somehow and this is done at IXP (internet exchange points: https://en.wikipedia.org/wiki/Internet_exchange_point )-- basically a huge building where separate providers can come together and exchange traffic.

I imagine that a peering connection in such a facility requires a heroic amount of maneuvering around 800 lb gorilla companies, but it certainly is possible if one has enough traffic and money.