Was it real Satoshi Nakamoto or someone guessed his password? 
I'd like to incline on someone guessing his password but that is highly unlikely, or someone may have sifted through the database and found something relating to Satoshi's account. Also, the post doesn't seem to pertain to anything at all, and is done to somewhat disturb the market for a brief period of time. I think this could be the same hacker as 2014 although there isn't sufficient evidence at all.
For context on the 2014 "hack":
Quote from: Robert McMillian
Dear Satoshi. Your dox, passwords and IP addresses are being sold on the darknet. Apparently, you didnt configure Tor properly and your IP leaked when you used your email account sometime in 2010. You are not safe. You need to get out of where you are as soon as possible before these people harm you. Thank you for inventing Bitcoin.