It's actually a somewhat bad and uninformed post, or perhaps the mistake (unclear whether knowingly or not) is to disprove a claim made by uninformed people.

No one with a good grasp of the space ever claimed that it wasn't possible on X11 to call into APIs to retrieve physical display size and map that to how many pixels to render. This has been possible for decades, and while not completely trivial is not the hard part about doing good UI scaling.

Doing good UI scaling requires infrastructure for dynamic scaling changes, for different scale factors per display within the same scene, for guaranteeing crisp hairlines at any scale factor, and so on and so forth.

Many of these problems could have been solved in X11 with additional effort, and some even made it to partial solutions available. The community simply chose to put its energy into bringing it all together in the Wayland stack instead.