The expression "canary in the coal mine" means "early warning sign" because miners used to bring canaries into mines as probes to check for toxic gases- the can

The expression "canary in the coal mine" means "early warning sign" because miners used to bring canaries into mines as probes to check for toxic gases- the canaries died first. The joke- or at least as I interpreted it- is that Gary Marcus himself would be the literal canary- i.e. he would literally die.