That absolute statement is absolutely false. Say, I have a chicken that can be feed from left-over bread or wheat. I can store the wheat basically indefinitely, so there is no spoilage. Also, I cannot use wheat directly, so it is unusable calories for me. But I cannot store bread for very long, so there is spoilage. Now, using the chicken, I can convert the spoiled bread into eggs and meat, useful calories for me.
Say 10% of the calories produced for me at a human go to waste. And I can convert them at 25% efficiency to usable calories via a chicken. Then I have just increased the yield of usable calories by 2.5%. In consequence, I need to buy less and thus emissions can be reduced.
Bread typically becomes "stale" (texture change due to internal moisture redistribution) before it spoils (usually by mold growth). Stale bread is still edible, and there are recipes that prefer it over fresh bread, e.g. bread pudding and bread sauce. It does not have to be fed to chickens to avoid wasting it.
And wheat can be converted to pasta, which can be stored for years, and can be easily made edible by brief cooking. You can reduce wastage of wheat to negligible levels, and chicken farming has non-zero overhead, so it seems unlikely that the chickens are an efficiency win if you take reasonable care.
It's pretty reductionist to look at this from a pure calorie input / output point of view. Keeping a few pastured chickens to feed food waste can provide pest control in a garden, an additional source of nutrition for people (which is a complete protein) and a rich fertilizer from their waste.
> In consequence, I need to buy less and thus emissions can be reduced.
You're not taking into account that, by virtue of being an animal taking in oxygen and creating carbon dioxide / other pollutants, the chicken itself is contributing to greenhouse gas emissions.
The question is, in real life, are the bonus calories we get from animal farming efficient enough so as to be worth those greenhouse gases? Given the massive GHG emissions of animal agriculture, it doesn't seem so.
Perhaps I'm misunderstanding your example, though: feedback appreciated
Say 10% of the calories produced for me at a human go to waste. And I can convert them at 25% efficiency to usable calories via a chicken. Then I have just increased the yield of usable calories by 2.5%. In consequence, I need to buy less and thus emissions can be reduced.