Nothing but the Btc foundation giving itself legitimacy. I highly doubt he actually signed any documents or was contacted in any way to get his name on there. Why would he support a foundation for a decentralized currency, ESPECIALLY if he went underground back in 2008 (I believe?) and the foundation was formed in 2012.
Also he did not trust many of the foundation founders(i.e. Gavin gave information to the CIA and so Satoshi would never talk to him again), here is the reason probably he disapeared.