The terms "right", "left" and "centrist" are relative terms, and apply to the current balance of opinion in a country. They originated in the French Revolution, where the "progressives" (the revolutionaries) happened to sit on the left side of parliament, and the "conservatives" (the monarchists) happened to sit on the right. Starting a tradition which has continued to this day, where those who are wanting to change society sit on the left, and those who want to keep it the same or roll it back to what it was when they were children sit on the right. There is no such thing as a right-wing policy or a left-wing policy, because as societies change the policies supported by the "left" and the "right" change also. In practice, the "right" tends to support those policies that the "left" fought for 50 years ago, while the new left now wants a different set of policies and disagrees with the work of their forefathers.
So RFK is a "centrist" in that he is not a nutty communist like Harris, and he's not a true freedom lover or protector of life either. In relation to the present set of political opinions popular in the USA, he's somewhere in the middle, and therefore a "centrist". 20 years ago someone with RFK's opinions would have been called a leftist. But for today, he's a centrist. Because politics has shifted. If it keeps going on the same trajectory in another 20 years someone holding RFK's opinions will be considered right-wing.
I am obviously painting a picture with very broad strokes here, please don't debate it on little details, some details will obviously contradict it. But the overall picture is correct.