I think their chart is skewed by Detroit classic muscle cars which jumped in value between 1995 and 2005 when gas was relatively cheap. They are seeing flat values since 2007 while Porsche and other exotics have risen in value. Lies, damn lies, and statistics.
https://www.hagerty.com/valuationtools/HVT/VehicleSearch/Report?vbe=21532
https://www.hagerty.com/valuationtools/HVT/VehicleSearch/Report?vbe=29769