I've said it before here and received a load of lols, Microsoft changed to a new SDK, or GDK, the new ones allows development of a game across all three consoles and the PC, all with the same code pretty much. The advantages here are obvious. Sony went with the same development tools they used last gen, albeit with some additions. The advantage here is obvious as pointed by Cerny in that speech he gave where he seemed like he was high. Sony's SDK is a lot maturer and their devkit kits were also in developers hands a lot sooner from what I've heard.
Essentially I think we'll see the more powerful Series X pull ahead, maybe even within a few months, these are extraordinary times mind you, so it may take longer. If 3rd parties still run worse on the Series X in a year or two, then we can talk!