I don't want to start getting super pedantic about what exactly he said she said as these threads always go on MN.. it's tedious.
But since you've asked directly... you said
Children that go to Nursery wouldn’t (usually) be happy to just be chucked into a room of strangers and get on with it. And no-one is suggesting they should as your post suggests.
And then
whilst those who’ve been attending Nursery seem to throw themselves into these things far more easily.
So you ate saying that in your opinion children who attend nursery are more comfortable in the company of other people (strangers at first) and more sociable/outgoing than children who stay at home.
That is the whole point of your thread is it not???
Re: attachment theory, it's not my assertion, it's what the research suggests.
OF COURSE most children who attend nursery have secure attachements to their parents - I'm not saying they don't. The two are not mutually exclusive. But that's the whole point. There are so many variables you can't just say that nursery before 3 is benificial as is the whole point of your thread.
For DC with loving involved engaged parents who are placed in a quality hildcare setting with a key worker (for the gold to attach to), child led unstructured free play and lots of outdoor access the potential negative effects will likely be minimal.
For children from disadvantaged backgrounds with chaotic, abusive, home lives where they have been unable to form a secure attachement then good quality childcare can be benificial.
That does not mean that it is benifical above being in a loving, responsive home environment full time, as is the assertion in your OP.
Please read the links I posted above - the leading researchers in this field explain it better than I can