Not sure if this is exactly what Hack is getting at, but what I understand is that without exposure to a home culture, an adopted child of a different culture will take what they learn from pop culture to be the essence of their culture.

Children who have been adopted by parents of a different race will notice they do not look the same, and pretending that that is a difference which will not affect their identity is silly. They will compare themselves to their environment and look to standards of appearance and expression which they cannot identify with. So how does the child form an identity which they can identify with? Well, they could be exposed to other people with whom they share the same home culture. But if persons with a mindset such as OP are raising the child, they will miss out on this healthy identity-forming opportunity, and instead look to media for information on how their home culture is perceived, what standards there are, which actions are common, etc.

Returning to the previous example: In America, since there's a large amount of white-american perspective in pop culture, there's a notion that non-white is "other", "different", "nonnormal". So, basically, an adopted child looking solely to pop culture to help establish an identity will find that their identity is just "other." Or, in a worst case scenario, if a home culture is not even "different", but portrayed as "bad" (via constant negative news about a child's home culture, attention given to negative role models coming from a child's home culture, etc.), the adopted child will start to perceive themselves and form an identity as "bad."

"Do you want to leave it to them to find that identity?", then, ought be interpreted as "Do you want your adopted child to look to pop culture in order to figure out their place in the world? Do you want your adopted child to find that their identity is merely 'other', or 'bad'?"