Nooooo, that's not what I am saying. Take a closer look at the Melian Dialogue. First, we know it's an important speech in the narrative because it's not a public one, and it's in a dialogue format, not a dialectical argument. Because this was an account a) in private and b) after his own exile, this is one 'speech' Thucydides himself crafted.
In that dialogue, the Athenians scoff at justice as an universal truth, scoff at the Gods and in other ways indicate that they are immoral (their disregard for what is right, what is traditional, their insistence on violence, their greed). The Melians plead for their homes arguing what had previously been argued and accepted as valid argument (justice as an universal truth, rather than abstract concept) by Cleon, no less (who, remember, Thucydides hates - but nevertheless has speaking for the truth because it's an important point). The Athens crush the Melians anyway, and look at the prose there.
The dialogues are always Thucydides way of representing the major questions and his own impressions of the answers to them. At most, you could argue that he doesn't present a clear cut answer to the questions, but I think it would be difficult to argue that he was the first relativist philosopher.