I think the point is that at some level of granularity, it's an announcement that you are currently working with the authorities, which is expressly forbidden in the NSL.

There's some point at which a court will decide that you aren't complying, and stepping over that point is bad for your business. Since that point is very fuzzy, it's a dangerous thing to play around with.

Exactly. They could even order you to keep updating it. In the Lavabit case, the FBI asked and judge agreed to compel Levison's silence + lies if they got the key with reason being his business wouldnt suffer if customers were lied to about their privacy. That argument and a judge agreeing with it are worth remembering in discussions like these.