...can you clarify this?

Looking up 'exculpatory no' implies that the matter was clearly settled in 1998 by the Supreme Court, which decided the doctrine is wholly invalid and the obstruction charge can be applied.

If there's iffy case law here, I'm not finding it successfully.

I'm not a lawyer so don't trust my take on what's safe to say to the cops.

I feel like I saw a recent exception to this, but even if I didn't:

1998 is pre-9/11, pre-TSA, pre- the large riots of the 2000s and 2010s like ferguson and occupy. It's pre-snowden, pre-aaron swartz. It's pre-iphone which means its pre every case about recording cops in public. It's pre stop and frisk.

Criminal justice has changed a lot since 1998.

I don't know, I think the border is a no-man's land. They can pretty much keep you in limbo as long as they want.

Edit: Yes, US citizens are allowed to ask for a lawyer (at the U.S. Border). But, the 4th Amendment is mostly out the window.

That's not really true. As a US Citizen, on basic legal principle, I believe that once Immigration has established you are a US Citizen, they have to let you leave unless they suspect you of a crime.

Customs is sort of a different issue, they can go through your physical and digital belongings and search you.

They have to let you in, but they can confiscate everything you have with you at the border crossing.