"We are currently optimizing the Xbox Series X|S version to meet our quality standards, so it won't release simultaneously with the other platforms," the developer wrote.
I mean, while it's possible they have problems with the entire XBOX platform, it's probably the underpowered Series S again, which needs the extra optimizing.
Certainly a gamble from Microsoft to tell people all the same games will run on both X and S.
It'll only become more AAA titles in the future, that would rather not optimize for the S...