Pretty good read. They bring up quite a few good points.
IPPC reports are too conservative because insufficient attention has been paid to the importance of tipping points, feedback loops, and outlier events forecasts. The nature of consensus building has tended to marginalize more extreme scenarios.
Humanity already has a non-zero probability that wars and general political trends will reverse technological progress and waste the resources needed to adapt to climate change and transform the economy. Then the RPC8.5 scenario might be too optimistic, not too pessimistic.