Given that the subpoena itself contains language such as "YOU ARE COMMANDED" (sic), probably not, but I imagine Moxie asked the ACLU lawyers before making it public.

Given that the following sentence says "If you nonetheless plan to disclose the existence or nature of the subpoena, please contact the Special Agent identified above first".

I suspect it might not. I don't know why this additional information wasn't quoted by the parent comment.

I don’t have any experience with Homeland Security investigations but I know when the SEC begins major investigations they’ll often request the subject of the investigation voluntarily waive attorney-client privilege. Who on earth would do that? Well, just about everyone. Of course they can’t force a waiver of privilege but the implication is that things will go much better for you if you do. The investigation (and subsequent punishment) will be much less painful for you. I suspect there’s a similar implication lurking behind this polite ask as well.