SD didn't have to trade him (or draft him for that matter), but Archie didn't dictate either. It was Eli who didn't want to go to SD, not Archie not wanting Eli to go to SD.
And, let's not forget, this started as a private conversation between Eli, his agent, and SD. It was SD who aired it out, and Archie came in the picture to support Eli.
Coincidentally, Eli has said one of the things that went into that decision was how the Chargers were treating Drew Brees at the time.
But it certainly wasn't Archie dictating where he wanted Eli to play.