His account in p2pfoundation.ning.com has been hacked before, in 2014:

http://p2pfoundation.ning.com/forum/topic/listForContributor?user=0ye0gncqg772o

"Reply by Satoshi Nakamoto on September 8, 2014 at 23:10
Dear Satoshi. Your dox, passwords and IP addresses are being sold on the darknet. Apparently you didn't 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."

The same could have happened to his email account. If he wants us to be sure this is him, he should sign the message.