Another possibility is that Paul's writings have been heavily redacted and catholicized. Seeing "visions" was more common two thousand years ago than it is today, which is to say mental illness wasn't treated as such or "seeing visions" was almost just a figure of speech. Dreams were considered visions, too, and the imagination was a producer of visions. So Paul might have been saying only that he imagined the whole thing, but he'd have phrased it in a way to save face and to boost his authority.