Although scientists have proclaimed that this is the world's most stable clock, they do not yet know as much about its accuracy. This is a subtle but important difference: The ytterbium clock has demonstrated incredible stability of measurement -- it always measures a second in the same way -- but we do not yet know if what it is measuring is a "true" second.
So, we'll have to wait to find out whether these clocks could be the most accurate in the world. More research is needed. It's a story we hear time and time again.