They were, by politics, bound to England as a country, but many colonists considered themselves to be an independent group as they were treated poorly by English natives, English troops, and English politicians. I think the quote is talking about when Parliament began enacting the Intolerable Acts, imposing more restrictions and sanctions on the colonies, and the colonial mindset. As the Acts were put into effect, the colonists became more strongly bound to England through way of politics, but became more American, or independent, in mindset. The quote makes sense when you analyze it, but not if you are simply talking polical aspects.
<---- History major/geek