Not that outlandish... it's directly inline with energy density claims, Model 3 size considerations, lower cost of battery packs from the Gigafactory, and an extrapolation from the mile/kWh achieved by the Chevy Bolt (even though the bolt has motor with a permanent magnet making it slightly more efficient). 238 mi / 60 kWh = ~3.967 miles/kWh
If driven conservatively a 100 kWh battery pack might provide nearly 400 miles of range in a vehicle lighter than the P100DL. We still don't even know the range of the future Model S 100D. I'd guess a Model 3 would be even better due to less drag and less weight.
This boils down to if Tesla is actually willing to fit a 100 kWh battery pack to a Model 3. It's not technically impossible.