I’ve been thinking for awhile that an OOT release in 2017 is inevitable, and I still think that’s the case, but I don’t think the way they’d announce it is through Peter Mayhew’s twitter feed.
It’s possible that it won’t actually be Mayhew’s announcement. It might be he just knows when Disney plans to announce it and is doing his own goofy thing on Twitter to coincide with it. My bet (assuming it actually is an OUT release) is that Disney does its own press release and Mayhew tweets a link to it when he gets to the end of the script.