These studies tend to always rely on a perfectly balanced grid without any extra capacity to find these massive seasonal differences.

Add a bit of extra capacity to the wind/solar installations and the battery figures usually plummet.

This is for a single home off-grid, meaning solar over-production is already implied. You need enough solar available to charge that 1MW battery in time for it to be useful during those seasonal differences which is going to be multiples of your peak summer generation.