You cannot prove with any certainty that there were no materialist atheists in ancient history. However, in rebuttal, archeologists have found ancient tribes completely devoid of any religious artifacts whatsoever to metaphysical strains from Mahavira and Buddha to written evidences in the Greco-Roman era such as the elaborate critique of atheism in book 10 of Plato's Laws in which he asserts atheism is rife in Athens that they existed.
Which shouldn't come as a big surprise since the modern humans of antiquity certainly were able to reason and make metaphysical choices like us.
Now regarding the false charge of atheism to first and second century Christians by
some, as Justin said in ‘The First Apology of Justin’ before Caesar:
“For not only among the Greeks did reason (Logos) prevail to condemn these things through Socrates, but also among the Barbarians were they condemned by Reason (or the Word, the Logos) Himself, who took shape, and became man, they who did such things as these are gods, but assert that they are wicked and impious demons, whose actions will not bear comparison with those even of men desirous of virtue. Hence, are we (Christians) called atheists. And we confess that we are atheists, so far as gods of this sort are concerned, but not with respect to the most true God, the Father of righteousness and temperance and the other virtues, who is free from impurity.”
The Oxford Handbook of Atheism states, "There is, however, no evidence that the turbulence of the Jews was ever attributed to atheism, rather than to the bellicosity of their ancestral God."
Obviously Jews, Christians, and ancient pagans and atheists who took the time to learn what Christianity actually taught understood that Christians were NOT atheists though
some first and second century Romans and Greeks didn't.
With respect to that ancient Roman era, as the Oxford Handbook of Atheism states, "In the four centuries down to the beginning of the Roman empire in 31 BCE, disbelief in the existence of gods was a recognizable if rare stance."
As for Diagoras, obviously it's not possible to interview him; however, Diagoras wrote a treatise under the title of apopyrgizontes logoi (literally, destructive considerations) in which he attacked the belief in the gods and stated that he lost his faith in them.
You cannot state with any certainty that Diagoras was an atheist in the sense that we mean the word today. Remember that even Christians were designated atheists in antiquity. It means only that they held a view of God that was different from the official belief. None of Diagoras' writings have survived. We don't know why he was called an atheist. It may be he did not deny the existence of the gods at all. He may only have denied that paying them homage provided any earthly benefit, or it may be that he denied the official Athenian cult. We just don't know the reasons the charge was laid against him.
No, I am afraid it is not obvious at all that early in antiquity there was anyone whom we could truly call an atheist. There were no scientific explanations for where the world and life were derived. If there were no gods then there was no explanation for the origin of the universe. Furthermore, in antiquity the support of official cults was considered necessary to the well being of the state. Anyone who did not comply was designated an atheist. That is why Christians stood accused. Can you say for certain something similar was not going on with Diagoras? You cannot state with absolute certainty that he was an atheist as we use the term today.
