Liquid smoke can be produced two ways, chemically and distilled. Distilled smoke is made like like perfume essence, hot evaporation chamber, arched chimney, cooled condensation chamber. Natural is an abused food label, so we can't conclude that natural means no chemical processes are used. Wild Planet has also had some transparency gripes about using Vietnamese farmed sardines but labeling them Pacific. FDA requires them to put aqueous natural smoke on the ingredient list to call them smoked sardines without actually smoking them.
So you can presume ethics even though they falsely represent Vietnamese fish, or call them and ask how the smoke is made. My instinct tells me it is just as likely to be compliant as it is to contain nitrates.