SC 1.4.11 has this exception: where the appearance of the component is determined by the user agent and not modified by the author;

Yet guidance in the understanding document says this:
"Where the focus style of the user-agent is not adjusted on interactive controls (such as links, form fields or buttons), the default focus style is sufficient."

If you read the bit about focus style it seems to say that only if the focus style was modified then you have check for contrast -- whereas the exception indicates once you have made any change to the appearance of the control then you are responsible for ensuring contrast.