Its up to genes for the shape of muscles.
Also, the muscle fibers/heads of the chest, are going from the shoulder joint to clavicle and sternum etc, in a someway horizontal line if you know what I mean, so there can't be contracted the one edge of the fibers, for example the fibers near sternum or clavicle ("inner" chest) and the others near shoulder joint ("outer" chest), stay inactive.
With specific exercises and angles, you just emphasize areas (for our example upper chest, not inner or outer), and not isolate them from the whole muscle group.
Also its important to use angles, not so only for emphasizing, but to work the body and the strengths of it, in all planes (superior vertical like overhead pressing/vertical pull ups, incline like incline pressing/angled pull ups, horizontal like flat bench/horizontal row, decline like decline pressing/45 degree row and inferior vertical like dips/upright row for example)
Thats what I believe and my opinion.
So for me its a myth for the inner or outer chest thickness (there is no inner/outer heads for chest, just clavicular, sternal etc or if you want, upper, medial, lower), but not exactly myth for emphasizing particular heads of a muscle.