There are now many internet backbones. Back in 2000, when that page was written, the original NSF backbone was still the major one. As stated in the full detail, it utilised leased telephone lines from major comms companies.

However, that was before DSL. The DSL backbones are now mainly 'cable'. The largest is apparently owned by Cable & Wireless and includes some 300,000 miles of underwater fibre optic cable including 6 transatlantic and 8 transpacific cables.
This backbone was primarily created to link the worlds ATM networks together.