A more charitable version of their argument would be that reading isn't a worthwhile activity by itself. It has been marketed as such given the context of screens, but it is not magically more stimulating on its own than most other activities, nor are Twitter and co. magically unstimulating on their own. Plenty of people torture themselves with trying to start a reading habit when their heart isn't in it, and often gain very marginal benefits out of it compared to other forms of light entertainment they could find more enjoyable. For people who do enjoy it naturally, there is obviously nothing wrong with it. But I notice that online platforms discussing books are rife with anxiety on the number of books read, the "productivity" of it, and are filled with articles about the benefits of reading.
Essentially, the attitude we have towards books is a form of cargo-culting. It's not the reading itself that is important, but getting exposed to new thoughts, knowledge, and perceptions and synthesizing them over time.