But at high latitudes, this would require either unaffordable amounts of batteries, or so much solar that enormous amounts of it is being wasted during the summer. So why not store instead of waste that excess?
The alternative for those high latitude areas would be wind, which is available in winter. But then you have to deal with how you cover the rare dark-calm extended periods in winter, so you still need a backup supply (like e-fuels in turbines), or else batteries cheap enough to cover those (maybe a couple of weeks). Iron-air batteries might be good for that.
I'm guessing the 400 C is just because they have customers who only need that. They should be able to get to 600 C pretty easily; that's the temperature limit for cheap steel before creep becomes important. They don't want to use firebricks since the scheme depends on the storage medium being extremely cheap, which means dirt (maybe $1-2/ton if sourced locally). Seasonal storage requires extremely low cost storage media.