Take it with a huge grain of salt. He was former ign editor. Here is his tweet.

Jeremy Conrad ‏@ManaByte

RUMOR is that Insomniac will show Ratchet at the Sony event as it's well after the release of Sunset Overdrive.

Source:

Used to follow the guy on twitter, but he was on the weird side... but he sometimes did find out stuff that turned out correct.

Sunset overdrive has a season pass, so they'll be working on that for a while after. You can see on Insomniacs website that It has only just gone gold. To announce this so soon would not be clever, as the game won't release until 2016 at the earliest.

It's releasing next year. Insomniac does not just have 1 team. Just so you know.

Well it's time to show something anyway, as the game should be releasing soon, so that's a safe guess.

We did know that R&C footage was coming by the end of the year.

...the movie.

Coming to 2015 annoucned at E3.

Would be about time, the game's a summer 2015 title after all.

Insomniac's been working on the Ratchet game for awhile now, along with the production of the movie.  It's supposed to be out early mid next year, according to whatever show Sony announced it in (Gamescom, I think?)