In your home, when you use a power cord, it is wire surrounded by plastic "insulation." The insulation is there to keep the two voltages separate (i.e. 120 Volts and ground or zero volts). Actually on a typical home power cord, you have a current wire at voltage, a neutral wire at zero volts, and an equipmen ground wire, but that is more than most people want to know.
For a large three-phase transmission line one normally has three conductors (one for each phase connected "delta." --some transmission lines also have a small 4th wire or static wire for lightning protection on the very top---Distribution lines in your neighborhood are usually 3 phase wires and a neutral connected "wye." but again, more that most people want to know). These three conductors on the transmission line are separated by air.
Under most conditions air is a pretty good insulating material. It is inexpensive. However, there are a couple of problems with smoke. As the article points out smoke is mostly small carbon particles, carbon is a conductor. Carbon arc lights pass electricity through carbon as do carbon brushes on motors. The carbon in the air means that the good insulating material becomes a so-so insulating material. Unfortunately there are two problems with smoke.
The first is that the folks who designed the transmission line separated the phase conductors an appropriate distance for air insullation and not for a mixture of air and carbon. That means that an arch can flash from one phase to another.
The second problem with smoke is that the insullators at transmission towers or pole structures can be dusted with carbon powder (soot) from the smoke. When this happens you will get a phase to ground flashover at the tower.
Once an arc occurs, then the air gets ionizde (ozone). The ionized air is a much worse conductor than normal air. When that happens an arch can go just about anywhere as well.