kids & tech was my PhD area so I can give you a qualified take.
as with basically every study on the topic of "screens and kid brains", this one:
doesn't differentiate between types of screen-based activities
is correlational, not experimental, and doesn't control for obvious confounding variables
which makes it basically useless.
take, for example, the existence of tablet apps that are specifically meant for children and parents to use to read together. if "reading with parents" is the actual underlying variable impacting white matter growth, then the medium (screen vs paper) might not matter at all!
if "screen time" is a meaningful construct then so is "paper time", and it would be reasonable to collapse doing crossword puzzles, completing tax forms, writing personal letters, reading novels, and ordering from restaurant menus into one metric and try to measure its impact on brain development. but we don't do that because it's obviously stupid.