Dubious at best. If one has children they notice quite visibly that screen time makes their children more agitated during the days they have screen time. And screen time is as addictive to children as it is to adults, and we should consider the effects they have on their developing brains.
When science doesn't match your personal observations, that doesn't make the science wrong. It also doesn't make your personal observations wrong - it usually (in my experience) means that you're measuring/observing different things (assuming you're reasonably intelligent, which I am doing). In this case, "cognitive development and well being" are complex concepts that probably do not map exactly (or maybe even gross…
People aren't worried about screens in general, but social media in particular as well as other smartphone applications that are engineered to keep you engaged.
See Johnathan Haidt's literature review here for what an actually critical reading of the literature would lead you to reasonably conclude: https://jonathanhaidt.substack.com/p/social-media-mental-ill...