The 2/3 probably some guess based on how contagious it is and that initially nobody is immune, so as a pandemic progresses, it reaches a point where enough of the population is immune that one contagious person no longer passes it on to >1 other person, so it fizzles out on its own.
So based on no effective containment of any kind - which seems kind of unlikely. Even if it's still not under control in China, what they are doing with containment is surely having an effect - or cases would have kept increasing at exponential rates - and they are not - it's slowed down considerably.
There are a lot of wildy pessimistic opinions and forecasts out there. Take them with a grain of salt.