Ambassadors serve at the discretion of the President and he doesn’t NEED a reason to fire any of them. In this case and I am quite sure all cases like this a President relies on input from his advisers.
It is another nothing burger but I am sure the many concern trolls here will wet themselves (again)
Testimony from a disgruntled former bureaucrat like McKinley who resigned in a snit over a disagreement with Secretary of State Pompeo is about as biased and useless as anything you’ll find.