They're talking about the so-called skyscraper index, "a whimsical concept put forward by Andrew Lawrence," but which posits that a spate of skyscraper building

They're talking about the so-called skyscraper index, "a whimsical concept put forward by Andrew Lawrence," but which posits that a spate of skyscraper building precedes an economic downturn but it is a conjecture that hasn't been verified†. “A study by Barr, Mizrach and Mundra (2015) […] They find that there is virtually no relationship between the timing of record-breaking buildings and the business cycle. […] In other words, the study finds that extreme height is driven by rapid economic growth, but that height cannot be used as an indicator of recessions.” † https://en.wikipedia.org/wiki/Skyscraper_Index#Empirical_test_of_the_Skyscraper_Index_hypothesis https://en.wikipedia.org/wiki/Skyscraper_Index#Empirical_tes...