It seems clear that while the Supreme Court didn't identify a specific requirement that officers state that people being questioned may refuse to answer, it did state that their "conduct" must give people "no reason to believe that they would be detained if they answered truthfully or refused to answer".
It seems clear that the conduct in question failed this constitutional test, and was thus unconstitutional per standing court precedent.
Meanwhile, the Santa Cruz, CA police department is claiming that Homeland Security misled them about the nature of recent raids, which appear to have been partially intended to identify and capture illegal immigrants not suspected of criminal activity. The Constitutional implications of this are unclear to me; does misleading the government negate jurisdiction in some way? Can California refuse some federal immigration enforcement, on top of the refusal to report some information that some of its sanctuary cities already promise to do?
Luckily for us, not every possible government police or military action is legal or constitutional. For instance, the government may not monitor the content of individual phone calls without a warrant; government officials may not explicitly misrepresent their identities or roles in the course of investigating illegal activity; and government officials may not make people believe they are legally obligated to take some action, such as producing ID or submitting to a search, if they are not actually legally obligated to do so.
When Melania Trump became an illegal immigrant by violating the terms of her visa and performing paid work in the US, as all available evidence suggests, I am glad the laws and constitution were there to provide her some peace of mind that the government would not have carte blanche to use every means at its disposal, such as unwarranted wiretaps, false threats, false impersonation, and illegal demands to pursue and arrest her.
While I wish the Supremes had gone further and established an analog to Miranda rights, they did make it clear that not only are government agents violating your rights if they lie to you about them, they are violating your right even if they imply false information about your rights by their words or actions.
Routine violation of constitutional rights should be a criminal offense punishable by jail time, in my opinion. But I know the Supremes aren't with me there.