He was the Chief Commissioner of the Climate Commission, the Government body responsible with disseminating information about climate matters to the public. He isn't just some individual with an opinion. Frankly people who have a habit of making these sort of predictions are exactly the wrong people to lead climate advocacy.
I agree with you - he is a good self-promoter who wrote a book about climate change. It doesn't make him an expert.
Making concrete predictions based on modelling which isn't specifically predictive of shorter term climate fluctuations is not sensible, and harms the cause of promoting an understanding of the actual science in the medium term.