To me, I just seems that PS5 is going to be a joint announcement by both Sony & AMD and they need a place (and time) where they can sell their idea to the consumers/developers. I'm not sure how Navi is integrated into the PS5, but I think that coding for it will not be as straight forward copy as coding for the Nvidia variant. There may be some porting involved and features which may require extra work for developers who may not want to participate.
I'll straight out say it that I'm an AMD/Radeon fan, and I think AMD is bringing a lot of innovation to the table, but it's been very difficult for them to compete against two big giant brands that have manipulated the market for quite a while now; hence, I think PS5 is a gamble for Sony and AMD to show innovation in a way people can accept it, because if the in-house developers actually take advantage of the AMD/open source (Vulkan) technology, and it might incite third party developers to follow.