In the absence of basic parenting, yes - that's an important function the setting serves. But it's not 'preparation for 1st grade', it's minimal damage-mitigation.
In the absence of basic parenting? The child is spending like eight hours away from their parents. Of course it's going to have an impact, parenting present or not.
I'm pretty sure the commentator was talking about the other case, social skills can be learned outside of pre-school and kindergarten - when basic parenting skills are applied.
Pretty much, yeah. If those kids that aren't going to kindergarten have exchanged that time for neglect - sitting in front of a television for an eight hour stretch, for example - then they will certainly be worse off for it. But the article didn't demonstrate or investigate that, it went straight from "less enrollment" to "less prepared", as if kindergarten is the only realistic place to learn such important skills as "basic respect" and "don't hit people".