I suppose it's possible that the truck has a newer V6 swapped in (or even an older GMC V6) but I suspect it was just reflex to say that instead of saying straight 6. It probably doesn't apply to Craigslist but I think some auto sell websites that have auto fill entries only allow you to enter V6.
Not really related to this conversation, but I spent a summer during college back in the middle 1970s driving trucks for a local manufacturing company. One of them was a mid-1960s GMC cabover box truck, with the world's most worn out V6 engine...that thing could hardly get up to highway speed, and if there was a big hill forget it.
It had about zero compression, I remember that when I turned the key off it would take seconds for the motor to finally stop spinning. We had a second cabover of the same age but it was a Chevy with a small block, and it would run rings around the Jimmy.