My impression of "defensive pessimism" as used here is that it means imagining all possible outcomes, including very bad ones. Another way to talk about it is looking for "unintended consequences".

Personally, I separate thinking about possible outcomes from the actual anticipation of which one is most likely (the latter which I refer to as pessimism/optimism/realism), although you could certainly make the case that optimists most only look for good outcomes and pessimists mostly only look for bad ones.