https://criminal.findlaw.com/criminal-charges/extortion.html
"Most states define extortion as the gaining of property or money by almost any kind of force or threat of violence, property damage, harm to reputation, or unfavorable government action."
My IANAL interpretation of that would be extortion wouldn't apply here. Because my security deposit is legally mine, and they were holding it on my behalf, I'm not gaining anything. Demanding my own property back is not the same as profiting. If I had demanded my deposit back plus $1, then it would be extortion. Maybe. Again, IANAL. Fortunately the statute of limitations makes me safe, as this was long ago :)