In pair programming as I learned it and as I have occasionally experienced it, two individuals challenge each other to be their best selves while also handing off tasks that break flow so that the pair as a whole is in constant flow. When it works this is a fantastic, productive, intense experience. I would agree that it is more than 2x as productive. I don't believe it's possible to achieve this state at all with a mismatched pair.
If this is the experience people have in mind, then it's not surprising that they think that those who think it's only for training juniors haven't actually tried it very much.