It’s quite simple, really: since you’re not a slave owner, i don’t belong to you, so the “threat” to quit can’t possibly be legally qualified as a threat (you never had the right to assume that I will continue to work for you beyond my notice period). It’s a contract negotiation, not a threat, so not extortion. The same way that “we will cut off the internet access if you don’t pay the internet bill” is not extortion from comcast, it’s just a statement of fact.
I think it all comes down to semantics, and the exact definition of "threat". If you define threat as warning someone that you will do something that will negatively affect them, then threatening to quit absolutely qualifies as extortion, just as Comcast threatening to turn off your internet service if you don't pay your bill also qualifies as extortion.
As I said in another post here, Wikipedia's definition of extortion is: ""Extortion, which is not limited to the taking of property, involves the verbal or written instillation of fear that something will happen to the victim if they do not comply with the extortionist's will." Getting your internet service is a negative thing, so and Comcast's will is for you to pay your bill, so I think it fits the definition of extortion here. As do all contract negotiations.
In summary, I think these definitions need a lot of work.
Actually, in some cases you do. But anyway, blackmail is not about telling secrets, it's about coercion under the threat of telling secrets. You know, kinda like how rape is about consent, not about preventing people from having sex.
