Preservation is largely unnecessary. If a garden provides say 1 month of food for a family, then that family buys less food for that month. Being self sufficient was possible, but takes significant labor and land.

This sounds plausible, but....you couldn't just "buy" more food into the economy in winter. You had to store it somehow.

A victory garden would let other farms concentrate on more storable foods though, if the gardens accounted for greens in summer.

But actual victory gardens had storable foods, I think.

That was already happening.

Large scale agriculture was already set up to do long term preservation. People normally eat storable foods every month. Bread for example generally does not last long, but it’s made from grains that could have been harvested 2+ years ago. Modern preservation methods can push that to 10+ years at minor cost.

That said, canning and other food storage was common. It was simply not a major focus of victory gardens. Otherwise the focus would have been on potatoes and other easily preserved foods not vegetables.