Got my car back!

Feels great to be driving the IS again. I was driving the NX200t as a loaner.

 

If it was the pinion seal on the front diff., the way it's located, facing the rear of the vehicle, the likely of something from the road hitting it is next to nil.
Additionally, the pinion flange has a deflector in the event something tried to make a suicide run at the seal, the deflector should also protect it. The gap between the deflector and the diff is so small, that if something did get in, it would be too small to cause any damage.
Typically that seal wears out for two reasons: the seal gets dry and hard from age, say 10+ years, or the pinion bearing is worn and the pinion shaft is wobbling as it turns, which elongates the seal.
Considering the age of your vehicle, I doubt the seal was dry and hard. And if the pinion bearing was gone, you'd also be getting some weird noises from the diff, which you never mentioned having.
The last possibility is just a defective seal...*stuff* happens