tl;dr: by using more spectrum in the coax cable can get pretty fast... So pressure from Google should get them to crank up the speed sooner than later.
As mentioned in the article, this requires a DOCSIS 3.0+ modem, and there are costs to rolling out equipment. Cranking up the speed isn't simply a matter of turning the dial on their end.
However you are correct that coaxial cable has plenty of room for improvement with a cost that pales in comparison to rolling out FTTH.
However you are correct that coaxial cable has plenty of room for improvement with a cost that pales in comparison to rolling out FTTH.