Understanding PPMs at the WTO

In the Peter Fitzgerald paper mentioned in the last post, I saw a reference to this sentence in "Understanding the WTO":

The WTO agreements are interpreted to say ... [that] trade restrictions cannot be imposed on a product purely because of the way it has been produced.

See p. 66.

I was surprised by the definitiveness of this statement.  I would agree that this is one view of the issue.  However, I think the opposing view -- that measures regulating the production process may or may not violate the rules, depending on various factors -- has support as well.