It's hard to tell if Paul was implying that he himself did not know if he himself was in the body or out when he encountered this "man". Would be good to see how it was translated, maybe I could find that online somewhere, will check.
"Out of body" is an old terminology as we can see. "Out of body experience" is a modern terminology that has been used to describe something that sounds very similar to what the disciples experienced, simply using their explanation of being "in the spirit" (conversely, 'out the body'), or the description Paul used here. How many different ways are there to say "out of the body", I can't think of many. None really, so it wouldn't surprise me if todays terminology is the same as then. Even derived from the original.
He doesn't know if he was either in his body or out and/or he doesn't know if the man he encountered was in the body or out. That seems clear cut. And he wasn't sleeping, or dreaming, apparently. He didn't say that. He classified it as a vision or revelation. There are examples in scripture where visions come in dreams, and they are explained that way. It wasn't described that way here.
The revelations shown to John, as recorded in the Bible, were shown to him when he was "in the spirit" (or out of the body)
Rev 1:10,
Rev 4:2,
Rev 17:3, and
Rev 21:10.
I am leaning toward the conclusion that these men were in fact experiencing these things out of their body, in their spiritual body. And would love to see any related scripture either pro or con, if any comes to mind for anyone.