Agent Engel would have been aware of all the relevant intelligence related to the mob’s intention and he would be responsible for informing the chief of staff and the President. It’s reasonable to argue he should be exempted from testifying because his communications with the President should continue to be privileged. If an agent can be called to testify about presidential behavior, then it could damage his ability/secret service’s ability to protect the President, any President. You don’t want a President “hiding” from his agents. That makes some sense. However, if Hutchinson’s testimony leads to the Agent being called and the secret service agreeing to his testifying, it’s BAD for Trump. The agent knows what went down and his testimony will be unimpeachable by Trump world. Trump may have not grabbed him, but everything else and more about the story will be confirmed.