Other studies discuss distraction, cyberbullying, bad diet/poor exercise, toddlers' sensory processing abilities, anxiety, and lost sleep. None of this covers stupidity, and much of it is not about teens.
The most interesting study you cited finds structural brain differences in preschool-aged children who spent more time on screens. It's still a stretch to make claims about the intelligence of teenagers based on the fact that babies who spend too much time on screens are, at age 3, less developed.
There's also a magnitude problem. Even if we assume smartphones do have some cognitive effect on teens, how can we know it's the only or largest one? You can't attribute anecdotes about kids being dumb to the presence of smartphones.