The real issue behind this, like often, is that what is bad for the children is actually also very bad for the parents, but they think they can get away with their particular dose: cigarette, alcohol, sleep deprivation, bad eating habits, not enough exercice, toxic env, etc.
It's the same with screens: it takes a toll on everybody, no matter the age.
It's very easy to verify: go on a screen diet, but be very active during it. Don't sleep more, don't eat better, don't try to be less stressed. Just remove as much screen as you can. You may first notice a withdraw period, depending on your screen consumption. Passed that, your energy goes up, you eat better, your social life improves, your ability to focus as well, and you are actually more efficient for tasks that can't be shortcuted using screen-based tools.
Now as a computer scientist, I keep using screen hours and hours every day, knowing that. That's the whole problem: knowing the cause is not enough to solve an issue. It's even harder since screens are at the center of a lot of work and fun related activities of our modern lifes. I even had to buy a dumb watch to avoid looking at my phone to get the time.
Of course, if you don't solve the issue for you, kids will see that you give them advices you are not following yourself. And I distinctly remember as a child that as soon as I noticed this about an adult, I lost respect for what they tried to teach me at the moment. Being young != being stupid.