It comes down to semantics of “kill”. I wonder if there's anyone can believe long work hours are the sole cause of mortality, regardless of whether they read the title.

The usual interpretation, long work hours are the major factor in the mortality of 745k people, is a valid interpretation of the research results.

To say it only contributes to mortality along with many other factors, without pointing out the portion of correlation, is to say nothing at all, which isn't what the report tries to communicate.