Methinks he meant that a GFCI would trip more reliably with the sink being well-grounded.
I really have no idea of what he intended to say.
Thought the same thing but if a toaster,coffee pot etc ( even with short cords ) on a GFCI protected circuit thought the GFCI would have a faster response time if sink was bonded. If a toaster that was plugged in but not on being they only have a two wire male plug ( no ground wire ) the GFCI might not trip and sink could be energised.
I see no way that the sink becomes energized.
If the metal sink has no connection to earth, there is no shock hazard and there would be no need for the GFCI to trip.
A bonded sink, in this application, only results in a short duration shock (the trip time of the GFCI) that would not even happen if the sink is isolated.