A long time ago, I took a nutritional science course and they explained food combining. I believe that the theory is correct. It'misunderstanding about it. Also, some of the terminology used is misleading. But, the truth is that most vegetarians don't need to worry about it.
Protein is composed of amino acids.
Each type of protein has a specific proportion of amino acids so human protein has a specific proportion. When we eat foods with protein, our bodies break the protein down into the amino acids. Later these amino acids are recombined to form human protein. Our body uses only as many amino acids as it can. Any left over amino acids aren't utilized.