Why are you so dead set on downplaying this incident? How do you even know the people in the video are actually feds if they're not announcing who they are and going through proper procedures to arrest someone? What's stopping some jackass in cosplay from pulling the same stunt?
The ultimate answer to this is; potentially being prosecuted for kidnap and impersonating police.
Most police powers boil down to exclusive legal defences that non-police can't use.
I realize that as social and political conditions become more polarized, it gets harder to practice the intended spirit of the site, but if HN is to survive, we all have to pitch in. That's one reason the guidelines say "Comments should get more thoughtful and substantive, not less, as a topic gets more divisive." "More divisive" is the trend of the world these days, and there's no way HN can expect to be immune from that.
Being that the parent comment is dead I'd assume that this comment won't be publicly visible but I'd like to give a deeper commentary that could be misconstrued by some here, as well take some thought to put together so I'll send that to your HN email.
Regardless of the above, I'm chagrinned and remorseful to generate non-positive dialog here.
I'm just not tremendously sympathetic to people who contribute to such an atmosphere of lawlessness and then report that their arrest wasn't entirely by the book.