1. As I mentioned, non-disclosure is very different from compelled speech. Compelled accurate speech is even held to a different standard than compelled lies. I have the legal expertise to tell you they are different. :)
2. This person seems to have missed choice b: "Do not comment". They are not compelled to lie, by the very law you cite. They are only required not to disclose. No court has ever held this to mean "lie when asked", rather than "say nothing when asked". If the government went after someone for not saying anything, that would be ... a tough case.
3. I am happy to admit the distinction between compelled lying and non-disclosure is, for some people, no distinction at all, but the law does make such a distinction.
What you say may be traditionally held by non-secret courts, but the revelations so far indicate the secret courts produce some astonishing rulings.
If the chap quoted in the Washington Post article was actively challenging the constitutionality of National Security Letters with the help of the ACLU, that makes me think he would probably have received reasonably reliable legal advice? Don't you think?
First, it is not secret courts who have produced any precedential rulings on the reach of NSL's (at least in the sense of binding any large group of people). It has been normal federal courts. Those that have ruled, have ruled the gag portion unconstitutional.
Here in fact, it says they verified he's the person through publicly available court documents, which must mean it's likely docketed in a normal federal court somewhere (the article pre-dates the FISC publishing their docket)
Second, there are two issues
I do not expect he necessarily received reasonably reliable legal advice. The people who participate in these cases are often not specialists, and often not highly knowledgable about the area (especially at this stage of the game, when things get to SCOTUS or something they generally are willing to engage more competent people). They are just passionate.
Past that, he was not quoted, he wrote the piece. You assume the piece is, for example, not using hyperbole. It does not say the government, or anyone else, has actually made these threats. It does not say what legal viewpoint they take that makes them believe this (and again, given the only gag orders to be challenged have all been struck down, it seems a bit out there ...). There are no details or anything else to support or verify the legal reasoning or implications for what he says.
This is an opinion piece, meant to support his case.
Reading it as an accurate view of the state of the law is, well, probably not a great idea (I certainly agree that reading it for the chilling effects part, fine. But to take everything he says as if his lawyer said that was the way it had to be, is a bit far)
2. This person seems to have missed choice b: "Do not comment". They are not compelled to lie, by the very law you cite. They are only required not to disclose. No court has ever held this to mean "lie when asked", rather than "say nothing when asked". If the government went after someone for not saying anything, that would be ... a tough case.
3. I am happy to admit the distinction between compelled lying and non-disclosure is, for some people, no distinction at all, but the law does make such a distinction.