Have they confirmed this or are people just assuming?

For example at the end of Sackboy trailer it says "Playstation Exclusive", Kena also featured that same ending as is also coming to PS4. So I really don't think know until we get given release data or confirmation from Sony.

It hasn't been confirmed but Jim Ryan's stance on generations makes it likely PS5 will have exclusives.  

Kena & Sackboy: A Big Adventure are third party studios. GT7, R&C, Horizon 2 & Demon's Souls did not have the "PlayStation Exclusive" disclaimer at the end. 

I wouldn't expect any of the stuff we've seen at the reveal from 1st party to ship down to the PS4, but I feel like we may see some titles that are coming out in 2020 to be tweaked for the PS5. Games like The Last of Us 2, Ghost of Tsushima for sure. I can also see third party games like FFVII remake jumping and we know CP2077 is already being prepped for the new systems (and will likely honestly be more built for them, much like Control isn't really built for PS4/XB1 and would perform better on the new hardware)