Is this a board member at your former or current employer? If it's the former, he's considered an insider so you did not disclose any information at all in the first place. If that's the case, you can ignore everything I say below and laugh hysterically in the face of any threats they make.
If it's a board member of your current employer, the information is almost certainly not confidential anyway. Confidential information will probably be defined in your employment contract or NDA. Odds are it reads something like "Confidential Information shall include all [material][non-public] information, written or oral, disclosed, [directly or indirectly,] through any means of communication [or observation] by [the Disclosing Party [or any of its affiliates or representatives] to [or for the benefit of] [the Disclosing Party]."
That's a pretty typical boilerplate CI clause, with the bracketed sections being optional elements. You should consult your contract to find the relevant clauses and see if the definitions fit your situation. There may also be a list of specifically included or excluded information, so pay close attention to those.
Some salient points from this typical definition, which may or may not track your own contract's defintion: Confidential information is disclosed by the employer to the employee. Your resignation was not disclosed by the company to you. Your employment status is public information. Your employment status was not conveyed to you for the benefit of the employer. It is not a trade secret. Thus it is not confidential, for any of those reasons or all of them.
More generally, though this law varies in its precise definition and application from state to state, NDAs need to be narrowly tailored to protect the reasonable business interests of the employer. There is no reasonable business interest protected by preventing you from disclosing your employment status with the company. It is possible that there was some arguable interest in preventing you from disclosing information about any inter-personal conflicts or doubts about the business that led to your resignation, but the fact of the resignation itself is unlikely to be material to any reasonable business interest. Any NDA that purported to cover such information would be overbroad and would be entirely invalidated in court. Or if you live in Illinois or New York or a handful of other states, the court may revise the contract to eliminate the overbreadth, but much to the same effect in your case. Keep that in mind, because if your former employer sues, they risk a court invalidating the entire NDA, freeing you to disclose any otherwise confidential information you are in possession of, so their lawyers may strongly advise against such a suit and all their threats are empty.
Ultimately, you have three options: ignore them and drop your wage theft complain, ignore them and continue your complaint, or consult with an attorney. If they have not sent any written communication on the topic, you could probably just ignore them and proceed with a wage theft complaint to your state's labor or employment agency without too much worry, but that depends a lot on the particulars of your case and your state's laws.
Which is why I would recommend consulting an attorney no matter what. It's possible that the cost of defending a lawsuit would be greater than your lost wages so it's not worth risking a complaint. It's also possible that your state labor department has mechanisms in place to protect you against retaliation if you file a wage theft complaint, so you may not have to worry about that. Either way, you need to consult with a licensed attorney in your jurisdiction who has access to all the relevant information to make that assessment. You won't get good information from people on the internet because we don't know where you are located and so we don't know the laws. We also don't have anywhere near enough factual information to evaluate your situation.
In any case, if you forgo the wage complaint and decide to ignore their communications to you so far, you absolutely need to go to an attorney if you receive any written communication from them threatening a suit, especially if it is on legal letterhead. Bring it and a copy of your employment contract/NDA to an attorney. The attorney will draft a response letter telling them to go to hell and will assist you in protecting yourself from vexatious litigation.
As for SEC and IRS issues, there are very probably no such issues implicated, at least judging by what you've disclosed here. I can virtually guarantee you about the IRS. They don't give a crap as long as you pay your taxes. They won't hold you accountable for any withheld wages unless you actually receive them, so just make sure you take that into account when filing your taxes. Pay special attention to your former company's W2s or 1099s or whatever wage documentation they provide you to make sure they aren't overstating your earnings.
As for the SEC, the only even conceivably relevant issue would be insider trading under rule 10b-5. If you are disclosing insider information to someone who is trading on that information, you may be legally liable for securities fraud. That would only apply if the company's stock is publicly traded. If you even suspect that you may be involved in something involving trading on insider information, you absolutely 100% need to talk to an attorney ASAP, regardless of everything else I've said here.
As for securities law more generally, if you were a director or officer, there may be some concerns about potential violations of duties to the corporation or shareholders there, especially if you continue to own a significant share of the company's stock, but you did not provide enough information for me to say any more. If you were a D&O, you absolutely need to consult an attorney. Similarly, if it is a member-managed LLC, you will want to consult an attorney. Those issues are far too complicated, dependent on state law, and specialized for random internet consulting.