Umm, a ketone is defined by the chemical group C=O with R groups (usually a carbon atom or chain) branching off of the carbon.
Doesn’t HAVE to be an amino acid when all is said and done.
It’s like the “all cats are mammals” and all that.
That had nothing to do with anything relevant.
All of the ketones in question (the ones made from consuming muscle you want to spare, not the fatty acids you want to consume) are created by the disassembly of ketogenic amino acids, notably leucine, isoleucine, lysine, phenylalanine, and a few more I can’t recall.
By supplementing with these amino acids when in a calorie deficit, muscle is spared because: (1) muscle doesn’t have to be broken down to consume fat and (2) the ready availability of the aminos facilitates the shift of the body away from the creation of glucose from muscle to the creation of glucose in fat.