In this article from InsideEV’s, it says the 250 kW charging speed is accomplished because of the higher amperage that it can take - 600 amps, which allows for the higher charging speed without resorting to 800 volts.
We saw the EX90 live at its unveiling in Stockholm and talked to some of the key people who created it in order to learn more.
insideevs.com
Other than cost, why would they decided to go with 400 amps, instead of 800? CCS chargers usually are limited to 350 amps, so you’ll need to find one that’s higher rated, to take advantage of the 250 kW.