Lets look at the issue in more modern English:
But if some of the branches were broken off, and you, being a wild olive, were grafted in among them and became partaker with them of the rich root of the olive tree, do not be arrogant toward the branches; but if you are arrogant, remember that it is not you who supports the root, but the root supports you.
The passage does not say those being addressed were in fact grafted in. The passage presents a conditional statement, if you were grafted in, do not be arrogant, but if you are arrogant, remember you are reliant upon actually being grafted in.