Because I knew of the said of the said Serpo exchange, I must apologize for my sect.
I don't know the semantical way of why the Serpo project ended up with those who were exchanged, said to have perished?
I do know that in a book dealing with the very early contacts said by author Frank Sculley in his first book, that a race was here in the 1950s 60s, that were energy beings.
They seemed to be able to wear a deceased human body, like it was a suit.They were said in Sully's book, to have made contact with him.
They came accross from what is described as, The Great Barrier. This is a belt barrier as you go towards the center of the galaxy, that no human can cross without dieing.
You can go across this barrier, but you must wear a space suit, with a energy variant compensator, in order to go there.
May be, said participants in the Serpo exchange, could have been afforded a protection type of wearable suit, that would have returned them safely.