If you watch the video, they say the DOJ reports a crew member was in the FC Lav.
I am gonna guess it was either the Capt or FO in there. It makes the whole situation more spicy. He's lucky there wasn't a FAM on that flight.
As for announcements, I hear it all the time as others have said "reserved for first class passengers unless a cart is blocking the aisle".
The only time I was given grief over it was DCA to SFO, and Nancy Pelosi was in 1C. The FC FA didn't take kindly from me coming up from Premium to use the lav, even though the cart was out. It wasn't until I was out of the lav, and the FC kinda went unhinged on me. I am sure the stress of Pelosi being on board with her entourage was part of it, but even Nancy just kinda rolled her eyes at the rant I was being given.
I didn't pull a DYKWIA moment, just said "sorry, the cart was out and I needed to go", let her harrumph at me one more time, and I went and sat back down.