I find myself understanding where both parties (those that believe it will be exclusive and those that believe it will be multi-plat) are coming from, but the main thing to keep in mind is that P.T. is its own thing.
It was a creative vessel to announce an exciting new project, but it's strictly stated it does not have direct connection to the upcoming game Silent Hills. Now, whether that is your typical Kojima-style misdirection no one knows- but what I do know is P.T. has been getting a ton of attention this past week.
When I go to Live from PlayStation (or whatever it's called) on my PS4, there are constantly people streaming this thing. People I didn't even know played video games were posting about it on their Facebook pages. There were people in these threads last week that were actually saying "Well, I think I'm going to go buy a PS4 for this". Hyperbole or not, it wouldn't surprise me if people were actually doing this. P.T. on its own has become a popular interactive means to freak people out- and it doesn't require a whole lot of time and/or commitment like a full game would.
What essentially amounts to a playable tech demo is giving Sony's console a lot of free press and now the majority of the people that experience it have been made aware that completing it awards you a teaser for a new Silent Hill.
Exclusive or multi-plat, I have no doubt Silent Hills will be widely associated with the PS4 simply because of the experience of P.T.