I don't recall why we once thought it was the second unit (maybe that changed with C3C?) but yes, the "Captured Unit" will be tradeable, besides being what captured population points - cities, settlers, workers - are converted into. The art assets for "Enslaved Worker" are indicated by the _SLAVE suffix in PediaIcons.txt, but to answer your question the enslaved unit is just a Worker in terms of game logic, so even if you capture a Settler which becomes 2 enslaved Workers and trade them back to their original owner, they become regular Workers.
Also holy craps you guys, I now get alerts for threads that I posted in a literal decade ago