He never lied
President Snow used exact words.
Yes, it is, indeed. But not in the way you imagine it.
KATNISS: How should I
SNOW: You should imagine thousands upon thousands of your people
dead. This town of yours reduced to ashes. Imagine it gone. Made
radioactive. Buried under dirt as if it had never existed, like
District 13 was “buried under dirt, as if it had never existed”—it was underground! Almost no one within Panem knew that it was still around.
President Snow is in fact being very honest here. To anyone who knew what had really happened, it would sound like he was being rather literal: of course District 13 is buried underground. To someone (like Katniss) who doesn’t know the truth, it sounds like he’s saying that District 13 has been destroyed, that it is “gone.” But only the last line applies to District 13, and it is literally true.
It’s a common theme in fiction for evil characters who are bound to tell the truth, but who want to conceal information or manipulate, to say things that are literally true but lead the unwary to assume what the evil character wants them to believe.
President Snow is not about to give Katniss critical information about the fate of District 13. But he wants to use it to make a point. So he tells her something that is literally true, but misleading.