The amount of misinformation on the net about theming in libadwaita is staggering. Global styling hasn't been disabled. There is nothing there that would really affect them in any significantly worse way than GTK3. That isn't to say they should do it, CSS theming will still be as broken and ill-advised as it is in GTK3, you can see that with the images at the bottom of the article.

"which align with an anti-theming petition that some of the team members support (stopthemingmy.app)"

To me this is a bad way to put it. I don't consider myself to be "anti-theming" and I agree with everything in that petition, except for the title which seems to be inflammatory and has contributed to some of the confusion and upset about theming in GTK. The title could be changed and the petition would be much better, I think. But it is probably too late for that.