Fair enough. Maybe a personal anecdote about how a non-economist could find a housing crash plausible isn't the best evidence for the availability of signals that something was wrong with the housing market. I can go through the FCIC report [0] and annotate to demonstrate, but their conclusion was the crash was avoidable. I can pile on more sources if that's desired. It'd be more entertaining to watch 'What I Learned and (Un-Learned) at the Financial Crisis' by Mark Blyth.
I object to Sumner's choice of time points in his blog, as they leave out the peak, and the historical norm. To observe that an already abnormal market is only a little bit more abnormal after ten years serves his point, but misses the story. That might be my meta-critique of economics.
[0] http://fcic.law.stanford.edu/
[1] https://www.youtube.com/watch?v=lhldDOp77QA