Of course they weren't Christians and therefore not prophets. But that doesn't mean that they wouldn't be able to undestand some face of the Logos. Because the Logos makes as much sense as 1+1=2, it is possible to understand SOME (keyword) of it through human intellect.
Plato, Aristotle and the Stoics did scratch the basis for absolute morality. It just conforms to natural law, which is set by God. And by that, it conforms to God's Will to some extent.
A lot of philosophers could have been saved in the harrowing of hell, but I think that's a stretch we shouldn't rely on.
We must be careful to not mechanize God, very difficult in the current era, and to think since they didn't tick the Christian box, they are 100% wrong in their philosophy.
God is not an autist and neither should you be one.