The stars appeared but they didn't see it.
In one way, I think it means they are gonna take sometime to be together again. But definitely they will be lovers forever since that's what the purple star means.
But with all that foreshadowing of illness, the scene turning into a portrait with sad bgm (in the trailer) ; I really can't tell what the writers are referring to as 'forever'.
Whether it means I will love you forever even if death do us apart implying an indirect happy ending for the characters (DW and YZJ loved each other, YZJ got revenge and had a happy life before death and later DW lived on to be a businesswoman dwelling on the memories with YZJ or an actual happy ending where YZJ gets the cure; I am really not sure about it.
Obviously I want the latter- actual happy ending!