That is purely a theory by one (yes qualified) person on one blog post without a single study behind it. I could easily argue that kids generally have short attention spans but social media just allows them to indulge in it more, and they will of course prioritize attention to that over other things. That is not the same as “it shortens their attention spans.” You need at least one study here - literally anything - linking social media and decreasing one’s attention span. The blog is focusing on how to reach kids and has one throw away non-cited line about social media shortening attention spans.

I should also point out that I also did a cursory Google search before writing the previous comment, and that was the only post I saw as well. The reason you selected it is because there was no other decent hit when you searched I imagine.