I read a bunch of pop psychology books (by supposedly respectably authors who were academic researches on this stuff) on child development when my kids were in that age. Weirdly, the author of this leaves out a completely relevant variable from the discussion: the child's age.

As far as I recall from my readings, daycare might be harmful for very small children (around age 2 and below), who are still very attached to their mothers. Above that, it is a matter of how large the group size is and how professional the daycare teachers are. Children below the age of 6 or so benefit from very small groups, because they don't yet have the social intelligence to navigate multi-person interactions. All children left on their own tend to develop anti-social behavioural patterns (such as taking toys from other by force, etc.), and a qualified daycare professional is supposed to watch for and weed out that kind of behaviour.