The problem is, precocious verbal skills do not mean a child has emotional development beyond that of the typical toddler or preschooler. I think a lot of parents ::::raising hand::::: sometimes expect too much from small children who are early communicators. That alone can lead to frustration and meltdowns.
As for turning it on and off--yes, that is true, and it can be a sign that the parent is doing something right! It shows the child is learning rules and the beginnings of self control. Of course the child will "let it all out" with the parent, because that is a "safe" situation. The problem is---WHY does the child have so much to "let out"? What have they been bottling up? Do they need help with coping skills? Is there a problem that needs to be resolved? Investigation is necessary.
BUT--in the mean time, the child needs to know what is appropriate and inappropriate behavior regardless of how they are feeling in the moment. Children need to know that some things are absolutely unacceptable. They also need to be taught how to appropriately express and deal with their negative feelings.
But a child with language is expected to use their words, not fling things around and throw themselves on the floor. If you watch, there is an element of deliberation in the fit-pitching. If you remove the audience, the kid will often follow said audience into another area. They are capable of self-control in other areas of their lives---good at school, carrying on at home for example--which tells me that they can turn it off and on.