Is this statement extortion?

It would be extortion if there were some kind of threat. But "legal action" is not a threat since anyone can undertake further legal action. It's not like they are saying you will be arrested or that you will be charged with a criminal offense. That would be over the line. Blackmail is forcing you to do something to prevent some kind of disclosure. I do not see that in this fact pattern. Nevertheless this seems like a scam especially if you have never done business with this company. Also the law firm may be in violation of the FTC related fair debt collection practices laws. Or they may be in violation of an ethical provision not to bring frivolous claims. You can report this to the FTC, your state attorney general's consumer protection office or to the state regulatory agency for attorney discipline.