Not sure if the "trying to surprise Sony" part is quite right -- common sense would suggest that, like with the Xbox One, Microsoft is just running behind schedule and will launch pretty hot -- but I've talked to a few third-party developers who said they were frustrated by the lack of info on Scarlett, especially compared to Prospero.
Na, they've let us know about the next box at e3. There's no chance of them being behind schedule either.
It'll be so close to specs and components with the PS5, I doubt 3rd parties will particularly suffer due to not having any access to the Scarlett kit.
Trying to keep something a "secret" doesn't exactly seem logical either. I'd put money on both of the companies knowing exactly what the other is building.