The light from most stars takes millions of years to reach us, so not only the present existence of these stars debatable, but so are the very concepts of the "present" and "existence".
Einstein's Special Relativity showed that anything that is far away in space is also distant in time. That is counterintuitive, but it seems to have held up in experiments. So the idea of "the present" meaning "at this same moment" now only works for something that is also "present" in the other sense, meaning "here."
I also so not understand how the concept of existence is made debatable by the distance and by relativity. Maybe the author claimed a little too much?