The full quote with context makes it pretty clear that the author is talking about crossing into interstellar space/leaving the solar system. It certainly could have been better worded though and should be fixed.

“And they have crossed into interstellar space, according to our best understanding of the boundary between the sun's sphere of influence and the rest of the galaxy. They are the first human-made objects to do so, a distinction they will hold for at least another few decades.”