That's right, Paul portrayed Jesus to them as being crucified. He's not saying the crucifixion itself happened before their eyes. He's saying they directly witnessed his preaching of the crucifixion. Paul talks only vaguely and spiritually of the dying and rising Jesus. The details he derives from Jewish scriptures (the gospels do the same).
If you think Paul's reference to the historical punishment of crucifixion commits him to the historicity of Jesus, do you think the fact that Greek myths say that Orpheus played a lyre and was buried and that the Maenads washed the blood off in the river Helicon means that that dying and rising god was likewise historical?
All myths refer to some concrete details, to avoid being philosophical abstractions that wouldn't appeal to the average person. There's more to telling the difference between myth and history than finding the odd detail or two.