Actually, that is a fairly good definition of isotropic pressure. After all, once you overcome rigid forces, what do you think is left? But if you are saying that you feel this should be the sole definition, then I tend to agree. Although I think differentiation might actually be better, I wouldn't split hairs about any definition that looks only at the internal physics and not the motion of the object. That's why I absolutely think that large moons should be considered planets, and I think that will be even more patently obvious when we start landing on them.