It is not extortion as there is no direct threat, other than impeding litigation. You have a good argument that the case, if an alienation action is ever brought is fraudulent as the affections of the marriage seem to be intact.
Some advice (from one that knows):
Do not communicate at all with the woman. No texts, no phone calls, no emails.
If they pursue a case, you will be asked in discovery “did you know she was married”? and “when did you learn she was married”? and “did you continue the relationship after you found out she was married”? And they’ll ask you for dates. If your evidence shows you had no communication with her after that date, it will be a plus on your side.
If you’re not going to continue a relationship (now or ever), cut off everything. If she DOES leave, yet doesn’t hook back up with you, I would think it would be harder to prove YOU were the reason she left.
Also remember, threats are one thing…actual papers served on you is another. If you get served, get a lawyer. A good one can blow a weak case out of the water. It will be expensive on their end to pursue this. If this woman tries to harass you or continue to contact you, change your email address, change your phone number (or block her if possible). Don’t talk to her in person…someone may be taking a picture.