Depends on what you mean by "Garden standards". No, a single plant isn't worth harvesting by itself, and one square foot would be barely enough for a single serving, but a typical 1 acre lawn, imagine a square that's just ~200 feet/60 meters on a side, is 43 THOUSAND + 560 square feet. The linear/square relationship can be really surprising.

Wheat yields like 50 bushels/acre, if I seeded my lawn in wheat I'd have 3,000 pounds, more than I could possibly hope to brew or drink. If I instead grew sweet corn, I'd have something like 15,000 ears. That's a lot of corn on the cob, and you've got to eat it within 24 hours of harvest for best flavor, and the harvest window is only a few weeks in early August.

No, you're not going to want to grow cereals one stalk at a time in a raised bed, nor grow it like a little potted oregano on your kitchen window sill. But it doesn't take that much cultivation - say, 10 rows of sweet corn along the edge of the lawn - to get more than you can hope to consume.

My experience growing small patches of grain is that migrating sparrows and sleeping deer will mostly wipe you out. These are relatively insignificant depradations for large fields, but if you only have 40 square meters they can destroy it all.