thanks for the reference!
It seems I misunderstood the description. I thought the focusable element in question was the .cc-mdl-close-element. But it indeed refers to not existing elements inside of it.
So I just read about it and even though the Google lighthouse description only refers to elements inside also the element with aria-hidden="true" itself should not be focusable.
A focusable element with aria-hidden="true" is ignored as part of the reading order, but still part of the focus order, making it’s state of visible or hidden unclear.