Two of my three had tablets, video games, and TV from the very beginning. Both taught themselves to read BEFORE entering preschool at 4 years old. Both continued to test far above both school and state averages in English language arts and math. My youngest doesn't seem to like devices, prefers more traditionally activities (coloring, doll houses, etc). She just started kindergarten and is more or less average, still learning her letters and numbers (no reading).
So I could say my anecdotal experience is the opposite of your own. Screen time seems to correlate with better testing outcomes.
Or we could both admit that it's not that simple.
I suspect there may be additional variables beyond just screen time that play into academics / test scores.