Good points...Congressman McCarthy and President Trump have no legal obligation to tell us what they talked about...
It is possible that either of them could be asked to testify under oath to some commission investigating January 6...To discuss the phone call...Then what? I don't think President Trump would ever agree to testify before such a commission...I don't know what Congressman McCarthy would do in a such case...
Then they site executive privilege which is a well know and often used phrase. Every president uses it to keep conversations with the president confidential, otherwise no President Trump could ever trust talking to anyone.