This is a fairly reasonable assertion, but this:
Does not follow, throwing and slinging are not the same thing. Throwing is a very intuitive action that can be made very effective through plain repitition.
There are a number of variations on this, tumbling thrown weapons might require training, other throwing tools such as an atlatl might require training, etc, but I haven’t seen a rationale for restricting throwing in general.
That’s missing the point that slinging and throwing are different, a natural overhand throw is quite close to being optimal, and it can simply be tuned with practice.
I’m extremely against the concept of training being necessary for throwing to be effective.