If you want legal advice, I suggest getting a consultation from a patent lawyer. They can help you determine how best to protect yourself better than any online forum, even one as smart as HN.
Too many things depend on your specific circumstances for generalized advice to be helpful.
I cannot even begin to describe how many times I've heard commenters here spout advice like they are experts in patent law when they simply don't have the most basic grasp about them. It's terribly frustrating and scary to see people taking this often horribly wrong advice.
Sure, some people here are right, you probably don't have much to fear, they're probably just bullying you, but please, please, please don't take legal advice from a random forum. That's like taking advice on rails development from a forum full of lawyers. They just won't know what they're talking about much of the time and it can be very dangerous.
You have a valid point but is it worth your time? Experts don't exactly rule the world. The asker just needs to know if this threat will destroy what he does and the simple answer is NO.
He has a plethora of options at his disposal, being the real deal in this situation. If this was a letter from a lawyer, then it would take a lawyer. Does it need to be more complicated?
If it really is a bluff, the best way to call that bluff is by having a lawyer respond to them.
Advertising that you have no legal representation is a very bad idea. They can find ways to get you to say things they can use against you, for example, by making things sound bad and thus convincing you to deny things you shouldn't.
And yes, I have had this explained to me by actual lawyers.
I disagree in this particular set of circumstances. Seeing a patent attorney is expensive and in this case unnecessary for the reasons argued above. This is nothing to worry about.
I'm sorry, but I disagree. I think that the description we've been given is likely incomplete and parts of this story simply don't add up. Maybe it is a bluff. Maybe it's some sort of scheme attempting to make any potential future infringement of the patent willful. We don't know, and those conclusions should be made by someone who is aware of all the relevant facts. Lawsuits can turn on the silliest little details, after all.
Legal consultations aren't that expensive, either.
Too many things depend on your specific circumstances for generalized advice to be helpful.