I think they use the 416 for ADR since it matches the mic they boomed with. This saves you the time of trying to match the sound of two different microphones.

If you record it in a dry room, it is possible to add reverb after. It seems like it's the reverb/echoes that gives shotguns their weird indoors effect that's undesireable.

They might have the microphone at normal/ideal booming distance/angle and they may have a reflective floor so it's just less work matching the ADR and non-ADR dialogue. That's how I'd do it anyways.

very informative thread.