In legal terms, one would call this a "statement against interest". If a politician says he does not care one bit about the welfare his constituents, it is reasonable for those constituents to take him at his word no matter how untrustworthy he is. It only hurts him if we believe it.

Iranian negotiators probably shouldn't believe him, because it's not against interest in that context. There probably just aren't any on Hacker News.

Correction: Trump probably isn't even a liar, but a bullshitter. A liar, at the very least, understands whatever he's saying is not true; a bullshitter doesn't care at all about the relationship between his words and the truth, he just makes noise.