Well – because you are CLEVER…
The conversation starts with …asking for a friend…  “So the tensioner is what pulls the terminal to apply tension to the wire. But what does the lock nut do that the wire tension isn’t already doing? Or is that just in case the wire loosens over time?”
.. sort of…
As presented, the statement, “…the nyloc nut is what pulls the bolt out to apply tension to the wire.” is correct. The hex nut has nothing to do with tension – think of it as a positional locking device. I.E. turnbuckles are available with (and without) locking nuts. The body of the turnbuckle turns – and by the virtue of the spiral of Archimedes – the “net” functional length of fitting becomes shorter (or longer) thus achieving ‘tension’. The locknut – used properly – dramatically reduces the likelihood of the tensioning resolving in an unwanted manner.
… and the need for locknuts comes from an interesting pedantic (but relevant) side point. The angle of a ‘standard’ thread pitch (60 degrees) … as seen in all versions of metric and imperial thread forms post the great conflict… (this type of thread is precisely defined in DIN 13 and DIN 14) is NOT ‘self locking’ so it NEEDS the locknuts – but the progenitor and predecessor thread form that is still available but often unappreciated (and is actually self locking) is BSW (British Standard Whitworth) and its angle of thread pitch is 55 degrees . So the WTFDIC is that large industrial (rated) turnbuckles use BSW as the thread form because it IS self locking and does not require lock nuts – thus reducing cost and complexity…