Satoshis gmx email account was hacked. With this email adress the hacker was able to change the account passwords of sites like the p2pfoundation, where he posted that message on. Months later he wrote an email to a trusted member of bitcointalk, saying that he has proof of Satoshis identity and that SN is still alive.
I have not seen convincing proof that the hack occurred before the months earlier p2pfoundation post.
There really is not proof (although gmx may potentially have proof one way or another). I think it is more a plausible explanation to the post.
I have seen video of the guy who was "tagged" as being satoshi and he appeared to not understand bitcoin very well and did not seem to have a very good grasp of how it worked on a technical level. If this person is satoshi then he is also a good actor
