PNAS' policies[1] say that experiments on humans must go through review by an ethics board before they will publish them, so it's especially strange that PNAS published this without any apparent review (none is mentioned in the paper). It's not up to the researchers to decide whether informed consent was obtained, it's up to the ethics board.
Having submitted to that journal before, I suspect the authors just ticked a check box that said something like "Relevant ethics approval was obtained for this paper".
[1]: http://www.pnas.org/site/authors/journal.xhtml (vii)