When defiant behavior starts happening frequently and consistently that's when they have ODD.
Most people will eventually try being defiant to an authority figure at some point in their development, and if it achieves a preferable outcome then they can get 'hooked' on it.
Whereas when an autistic person has a meltdown it's a stress response, more akin to a panic attack. They learn that they can get their way by throwing a fit, so it can become a habit. When a kid has a tantrum it's a somewhat social-manipulative attempt to achieve a specific desired outcome. It seems to me like it's very similar to the difference between a tantrum and a meltdown. I put this answer in r/PDAAutism, but I figured I'd share it here too:
