They didn't say unrestricted, they said locked out. As in, not accessible at all.
And you know what is also very hazardous to a developing person's mental health? Being left out of most social gatherings and interactions with their peers.
You can block your kids from accessing social media, but to succeed you also need to force their peers to communicate and interact with them in a unique and more frictive way. And you have no right to force someone else's kids to do that. So the reality is that without a lot of like-minded families that do the same, your kids are going to be left out of a lot of things.
So is it unrealistic to control your children's access to social media? No, not at all. But locking them out from it entirely as you stated, is a good way to negatively impact their social development.