F78: Overwriting default focus indicator

I have a question/comment about F78: Failure of Success Criterion 2.4.7 due
to styling element outlines and borders in a way that overrides or renders
non-visible the default visual focus indicator. The failure as it is
written seems to mean that the browser default focus cannot be overwritten
or turned off.  Does assistive technology use the browsers default focus
indicator to indicate focus to a user?
If so, then I understand that it probably shouldn't be turned off entirely,
but I'm having trouble understanding why the default focus indicator can't
be rendered non-visible, as long as it is replaced by something more
visible.

In my opinion, in most browsers, including both IE 7 and Firefox 2, the
default focus indicator is so hard to see that overriding it almost seems
necessary in order to make the focus indicator really visible.  It is
especially hard to see when a form element, such as a checkbox, has focus.

This Failure also seems to be in contradiction with 2.4.7 Any keyboard
operable user interface has a mode of operation where the keyboard focus
indicator is visible. (Level AA).  Reading 2.4.7, it seems that is OK to
override the default focus border as long as focus IS visible.  We also
allow an "author supplied, highly visible focus indicator" and C15 allows
the use of CSS to change the presentation of a UI component when it
receives focus. For example, changing the background color of a focused
menu item still makes focus visible even if there is no dotted border.

Does F78 mean that all of the sufficient techniques in 2.4.7 will fail?
Perhaps F78 should just say that shouldn't entirely disable the focus
indicator?

Thanks for your help.
Drew