I think that’s a misreading of the entire book. A black swan event is a rare, high-impact outlier that is nearly impossible to predict under normal circumstances, but is rationalized as having been inevitable in hindsight.
It’s the inflection point when our priors change.
And I have no idea where you got the "rationalized as inevitable in hindsight" thing. I've never heard of that in relation to black swan events before.
This flood is like The Big One in California. We know it is going to happen, it is virtually a certainty. The only question is when and where.
If glacial floods come as a surprise to you, it is only because you have been deliberately ignoring all the climate change warnings. So, a short recap: we'll see massive droughts, giant wildfires, huge floods due to rainfall, significantly more intense hurricanes, mass starvation due to large-scale crop failure, and low-lying countries drowning due to sea level rise. Oh, and add a high likelihood of the AMOC collapsing triggering a cooldown of the East Coast and Europe to the list.