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.
Sounds like hair splitting to me. He fell in or down a hole has the same meaning. I had no trouble determining his meaning. Your response was the one that was unclear. It made it seem that he did not understand the definition. I knew the meaning but I still looked it up to make sure there was not some bizarre other usage.