I can understand anyone doubting some "insiders" but Shinobi has been proven to have a solid track record, he's the one that initially leaked the stuff about Horizon Zero Dawn, he called the game Horizon, said it would have a a redheaded female lead character, that it was open world, developed by Guerrilla Games.
He talked about the mechanical animals, talked about the kinds of locations we've seen, tribal Humans and the setting.
He's obviously not nailing down a yes or no, but to be fair Sony has likely got choices, Level-5's game or games could be among those from other 3rd party devs and also 1st party stuff.
Realistically how many people do people think would know Sony's full PSX line-up at this point?
There's also the whole thing that these people don't want to give away who they are to the higher ups at the company they work for. If he gets sacked, then you get no more information on games early.
Personally I think any real legit insiders are actually a part of the marketing team and they're testing the waters to see if details about the games coming in the far or not too far off future are appealing to a market.
If Sony hears feedback they like, then they double down on those things in their development process or even make other titles based on information given by the community.
Of course people will have their skepticism, but there comes a point where you get to know who's given legit information and who's just riding on the backs of those that are doing that.
