"Absent any information about what kind of quality the kids in this study were getting, the findings are meaningless," said Susanna Cooper, director of communications for Preschool California.....Maybe these kids were in mediocre low-quality childcare situations. Were they overcrowded? What was the ratio of kids to adults? What was the training of the adults they were with?"
The point is they weren't home with their families.
Exactly. Being sent off (from the child's perspective) "on one's own" too early can harm a child by putting them in an unnecessarily stressful situation for which they are not ready.