Let's imagine that a Greek Philosopher, A, proposed a theory of matter that bares a striking resemblance to what contemporary experiments suggest is the case. The arguments for her theory, however, were at best dubious even by the standards of her time. Has A done enough to earn philosophical or scientific credit for the theory? To put it another way, how do we draw the line between lucky speculation and genuine insight in the history of ideas? Does such a line make a difference?
Perhaps a philosopher of science can address this question more cogently, but I'll make a few observations. You seem to be assuming that the theory of the ancient greek philosopher and the modern theory confirmed by contemporary experiments are the same theory, or posit the same entities, say. But it seems likely that the atomic theory of Epicurus or Democritus, say, is not the same theory of 20th Century physics, just as the ancient greek methods and those that Lord Rutherford used to confirm the atomic theory were also dissimilar. So I don't see how those ancient thinkers could have 'earned credit' for a theory they themselves did not propose. Nevertheless, we can still recognize the prescient nature of their insight into reality and refer to them as forerunners or precursors, if we wish. As for the difference between lucky speculation and insight, I am not sure that's a useful distinction to make.