Look, I'm usually super skeptical about rumors nowadays, especially when it comes to anything related to The Last Guardian. With that in mind, I will say that the following seemed notable enough to at least mention.

A few posts from a mysterious user on NeoGAF seems to indicate that they have some sort of knowledge that the rest of us plebeians do not possess. When asked in a thread on NeoGAF whether or not The Last Guardian will be shown at E3 2015, the answer was a very unambiguous "yes."

Usually when these supposed "insiders" post on NeoGAF, the mods require some sort of verification that they aren't just someone coming along and bullshitting everyone else. If the mods deem these people trustworthy after a PM exchange of information, they can stay. If the mods have reason to disbelieve them, they're banned. It's a simple enough system. As such, it seems that whatever proof this Zoetis person provided was enough to prove to the mods that the information they're sharing is indeed trustworthy, at least for now.

Of course, most rumors need to be taken with a grain of salt. This rumor, above all others, needs to be taken with the biggest grain of salt you can find. The Last Guardian has been the focus of many rumors for years now, so it's understandable that there is a huge cloud of doubt hovering over this latest rumor. The last official word from Sony is that they are working on The Last Guardian. The Last Guardian has not having been seen for about six years now and has been in development since 2007.