If you go off grid on purely solar your backup is not a battery, your backup is a genset.
It's a nonsense proposition leading to nonsense conclusions based on nonsense numbers. Very tiring.
With that said, from reading some articles about off-grid living, it's not outwith the bounds of possibility that people are looking at pure solar (or pure renewable anyway) off-grid living, for priniple-based reasons , rather than a more pragmatic rationale.
Back in the real world you wouldn't design your system that way. Instead, you would look at your worst case scenario - mid-winter, and calculate how many solar panels you would need to capture a day's worth of energy in one day. This would also tell you how large your battery would need to be. A prudent engineer would probably introduce a factor of 2 or so both in the size of the panel installation and the size of the battery to allow for a sequence of unusually dark days, but the multiples being talked about in the article are just ridiculous.