I don't think the suggestion was relating to what the US government can compel from users of Yahoo's service, but rather that they could compel Yahoo to provide the government access to that user's emails while simultaneously compelling Yahoo to deceive the user about having done so. The notion is that Yahoo could show the world source code and intend to use it, but when it came time to actually put it into use, the government could come and force Yahoo to use different code, written by the government, while also forcing Yahoo to lie to the world, claiming that it's using the code it had originally intended to use. Five years ago this might sound like a bizarre conspiracy theory, but now it seems much less like a question of whether the government would try than a question of how successful the government might be at forcing all the Yahoo employees who would have to know about the lie to keep it secret.