Outside of local edible vegetation, there is smaller game. And as you say, this is how many used to survive.
But beyond that, you can plant beans and have a crop in weeks. Even with a lack of pesticides and fertilizer, being able to plant a plant here, a plant there, will result in food not being eaten completely by pests.
And the ground is far more fertile, when not monocropping.
And yes, seeds can be reused, it's how farming still works today for many farmers.
To speak to this, where I grew up, many people would go into the woods, and plant a single pot seed here, another there, always close to trees etc, so overflies by police helicopters wouldn't see a crop.
Being rural, some people owned 1000 acres, and plausible denialability exists. Even if some went missing, the scatter method yields results.
And: compared to past centuries (or even pre-agricultural), a bit of help from modern society can save a lot of time. A lighter here, a $5 knife or cooking pot there, some discarded materials from a construction site, etc etc.
Doesn't make primitive life easy. But it's not hard to see why some people would prefer it over navigating the complexities (or stress!) of 'living in the fast lane' modern society.
you can plant beans and have a crop in weeks.And there's plenty of small game to hunt and trap. I don't see that Petrov did this, but I imagine if he was comfortable catching and eating a neighbors duck, he probably took a squirrel or two in his life. The natives would obviously fish, take all kinds of small game, even including rat.
So relative to our modern diet, maybe your point holds, but the California Floristic Province has plenty of plants to sustain humans. IIRC, the indigenous population density was some of the highest in the world.
It's a bit of a digression, but it's a subject I find interesting and rarely pops up in hacker news. :)
(Lived in the area for a long time, but not an expert at all). Did the anthropologist provide any details? I'd love to read their work