All else aside I mean how can they even claim to predict what an economy will do in 100 years anyway, it's going to adapt to complex higher order effects. Maybe climate change will increase GDP of everyone has to hire a worker to fan them with palm leaves.
Comparison: 100 years ago there was a global gold standard, Germany didn't exist, only a few people had cars, there were no computer machines no matter how rich you were, no transistors, no TV but many people listened to broadcast radio instead, stock trading was also for rich people and nobody was using the market to see how well the economy was doing, science fiction was about going to Venus because it was thought to be more habitable than Mars, protons had only just been discovered but not neutrons yet, and east of the Mediterranean was the Ottoman Empire.