I always understood it to be a precursor of danger. Warning miners of dangerous gas levels. The canary would die as a warning to get out of the mine. What is your understanding of the term? The BDI is a warning of possible future trouble.
His use of the term “canary DOWN a coal mine” means he never bothered to understand the term is “canary IN a coal mine”
It makes the author sound stupid.
And Yes a canary IN a coal mine was really used to help warn them of dangerous gas levels. If the canary died, they all got out of there quick.