Well the original rumour was that nVidia were using Samsung for their next gen but with the delays to Samsung's 7nm EUV it was then announced nVidia would be using TSMC for most of their production.
That doesn't sound like it was booked long ago. It sounds, to me anyway, like nVidia might have an issue getting sufficient capacity. Time will tell as allegedly there will be some details around Aug 2020 iirc.
Considering AMD can barely meet demand and nVidia traditionally out-sell them by a factor of 4-6 then I can see issues ahead for nVidia's next gen.
This video has some detail on TSMC 7nm capacity issues and who it is allocated to. I apologise for the source, I know his voice does some people's heads in
Also, the rumour of 50% more performance AND 50% lower power seems like a stretch. Convenient that was leaked just before CES when everyone thought AMD might tease big Navi. Wonder who leaked that BS rumour. Clue: He might wear a leather jacket