Like you said, the cooking of the inside is a function of the water vapor escape, which is a function of temperature and wrapping.
People elsewhere in this thread have said they have tried both methods, and commented on how when deep frying a whole baked potato, the escaping water vapor causes the oil to visibly, vigorously boil, but this doesn't happen with rosin, suggesting the vapor remains trapped inside.
While liquid water cannot go above 100C, water vapor certainly can. That water remains in the potato after cooking can just be re-condensed vapor. How the expanded gas would remain in the potato I have no idea.
My interpretation is that the difference from deep-frying is that something about the rosin method prevents as much water vapor from escaping. Maybe it's just typical cooking temperatures, and a deep fryer set to the same temperature as the rosin pot would yield a similar result, they just usually aren't set to that temperature.