That seems like an unreasonably high level of confidence to have when paired with no supporting material.
A Model 3 has a battery no larger than 83kWh. A BMW i4 has a battery no larger than 84kWh. This car has a 101kWh battery. Considering the cars appear roughly approximately equally aerodynamic, it seems reasonable to expect the car with the 20% larger battery to be able to go about 20% further.
It is not only about battery size, but also about efficiency. Model 3 and i4 have very efficient motors, specially the BMW (non-permanent magnet type). I have difficulties believing than Xiaomi can reach such efficiency levels in 36 months development. It is also important to note that those 20% extra kWh are heavy, and need to be accelerated to cruise speeds several times during normal-driving. I stand my point and don't think that the xiaomi will even reach 700km in best conditions (The BMW i4 made around 650km)
A Model 3 has a battery no larger than 83kWh. A BMW i4 has a battery no larger than 84kWh. This car has a 101kWh battery. Considering the cars appear roughly approximately equally aerodynamic, it seems reasonable to expect the car with the 20% larger battery to be able to go about 20% further.