This is explained in the post itself at length. Indeed, that’s kind of the central point of the post.
I was going to quote some relevant parts of it here but it would end up being just cutting and pasting entire paragraphs, so instead please refer to them above.
The briefest relevant snippet, I suppose, would be that the Swift 6 language mode is:
a targeted set of source-incompatible changes to the language. . . . Most features do not fall in this category, [including] major efforts that are underway . . . such as the ownership model or constant evaluation