He was a robot in the Jedi temple. Dudes been around 1000s of years according to some throw away dialogue about him training younglings. It’s also crazy to think the movies really only take place over the span of like 80-100 years based on Anakin’s age at the beginning of phantom menace +lukes age in the Disney trilogy. The imperial empire was around for like only 40 years.
A few days late and not OP but they aren’t referring to that, but the scene further along where Luke cuts Vader’s hand off as he is down on the ground beaten and the emperor to push him fully to the dark side says something like “finish him” in his sinister tone. And then luke has a revelation where he was about to kill his father and the roles were reversed from bespin. Luke then chooses to not kill his father and threw his lightsaber down. Then gets electrocuted by force lightning and causes anakin to redeem himself saving luke and “killing” the emperor.
I believe that’s what they were referring too.