I think they're just thinking longer term. They're looking to better batteries down the line. With solid state batteries the weight of the battery pack will go down because they'll need fewer safer measures, the energy density will go up, and the range will increase.
They won't have to change the platform much to accommodate newer batteries, so why engineer this small city car for a physically bigger battery pack now?
The Honda e is a similar small car with low range. They're also looking ahead to better batteries to improve the range without changing the platform too much.
The Mini is not a 'small city car'. It was in its original incarnation, with which the current Mini shares nothing other than it's name. A Toyota Aygo, VW Up! or Smart is a small city car.
They won't have to change the platform much to accommodate newer batteries, so why engineer this small city car for a physically bigger battery pack now?
The Honda e is a similar small car with low range. They're also looking ahead to better batteries to improve the range without changing the platform too much.