G.fast is an acronym for Fast Access to Subscriber Terminals and its main purpose is just that--to get higher speeds from digital subscriber lines (DSLs). It is modeled on Very-high-bit-rate digital subscriber line 2 (VDSL2) which was designed to get high-speed data over the copper telephone wires already in older buildings (VDSL2 was defined as standard ITU-T G.993.2 in 2005.) The only thing exotic about G.fast is its promise of 200-to-500 megabit per second speeds for DSL.
It seems the spectral efficiency without bonding (1 pair) is 5.7 bps/Hz, while with bonding (2 pairs) it is 10 bps/Hz. What's the catch? Distance (70m vs. 30m)? I would still not expect such a big difference.
I asked your question ("What's the catch?") to the Bell Labs experts and this is what they said: "Distance is one part of the answer; we're also doing some undisclosed optimizations to get the most out of the 2-pair/30m case."