Shinobi works for Wushu studios among other places. Guy has called a shit ton of stuff and was the original person to leak Horizon:Zero dawn before announcement.
Guy knows his studios and talks to Sony Developers on the regular.
I also know many people in many places. But quite often even devs working on a game don't know when the marketing team will show their game.
I mean, when I worked at Ubisoft some years I was part of their E3 'war room' (was literally called like that), in charge of one the community management/development posts that were sharing the stuff in social media and collected and filtered all feeback across internet asap to address any potential shitstorm asap. Only a handful days before being shown I knew what games were going to be shown, and many of them were hidden in codenames that we didn't know what were going to be.
Only the guys on top, the ones handling the event and part of the teams involved on the games to be shown knew that their games were going to be shown weeks before. In fact, there were last minute changes like games dropped a couple days before or other ones that didn't know if were going to have their demo on time or not until the last day/week.
As an example, you may remember that Kinect was revealed in a MS conference where there was an Ubisoft fitness game. Well, I know some people who worked on that game. 3 weeks before of the E3 live demo that was going to reveal the game, the game was supposed to be a 4 players game that tracked fingers, features supposed to be supported by the game, but the Kinect driver and prototype was super buggy and unfinished and Kinect wasn't able to track 4 people, in fact did have issues to track 2. And obviously never tracked fingers.
Well, the thing is that 3 weeks before MS sent them a new driver that basically changed how most of the Kinecttstuff worked and still worked like shit but they had to almost make from scratch the demo in 3 weeks. They dropped the 4P and fingers stuff and for the demo not sure but I think they also avoided the 2P thing for the demo, which ended being playable at E3 but not sure if the live conference demo was faked as the one of the infamous Star Wars game was.
Regarding Horizon, one of the producers who did work in my studio left to work on that game and go back to Guerrilla, where he briefly worked before in Killzone 2. It was when the game still wasn't known other than with the leaked? early artwork of a different Aloy fighting these dinobots.
I have friends and coworkers in many places. In most cases people keep things confidential. In some other cases, confidential stuff gets mentioned to friends, but we don't leak a shit. Because in many cases it's easy to follow the chain and know who leaked what because it's easy to know who leaked what, and that person gets fired and not hired elsewhere, or other people stop sharing secrets if know that you'll leak them.
Regarding people who run indie studios and their games get revealed in conferences/Nintendo Directs/States of Play/random indie streams with many games like the ones of the E3 days or in other big events, they tell us hours before that their game will be shown there. Sometimes I already know the game (in some cases I mentored/incubated them), in other ones I also discover what is their next game in the reveal.
I get the feeling that Square may not show more of Rebirth until after FFXVI is out.
I think it's likely that Sony may have a big show in what were going to be the pre-E3 days, the days where big publishers have their conferences: around the first week of June. Pretty likely at this event they'll show the launch trailer of Final Fantasy XVI, because it's released a couple of days before. Or they may reveal it separatedly, in a Square event.
The thing is that at that point this will be the end of the FFXVI promotion. And if in that event Sony makes a recap of the games to be released in PS5 during the next year/year and a half, make sure FFVII Rebirth will be there because it's going to be one of the main 3rd party exclusives of that period.
