The sentence is in fact a restatement and a strengthening of the 
sentence just before the last one:

 >>Adam Smith (1776, Book 4, Chapter 2) argued that::
(....every individual generally, indeed, neither intends to promote 
the public interest, nor knows how much he is promoting it. ...) By 
pursuing his own interest he frequently promotes that of the society 
more effectually than when he really intends to promote it. I have 
never known much good done by those who affected to trade for the public good."

Smith proved the conceptual possibility that, ethically, striving for 
self-interest would lead to the greatest interest for society. Point 
in question is that this striving should merely be directed to pure 
self-interest, and not to the greater good of society.

In the last sentence, then, he stresses that any extra deliberation 
of social import in the choice making process would be likely to lead 
to (in modern terms) a suboptimal outcome. This quote seems to 
represent the heart of Smith's ethics (rather than his economics).

Menno Rol