If the planet's parent star were still in another galaxy, it would be much too far away for us to find the planet. This is not to suggest that the discovery is insignificant, though–here, let's look at a couple of different hypotheses:

A) Planets might not form around every star, but they form around many stars, and that means there are billions of trillions of planets in the universe.

B) Planets can only form in the Milky Way galaxy, because of the unique conditions found here, so there are probably no more than about a trillion planets in the whole universe.

Now, up until last week, either of those hypotheses was plausible, because every planet we'd found was in our local neighborhood, and therefore was formed somewhere in the Milky Way, even if its star later got dragged around and mixed with stars from other parts of the galaxy. This new planet is also in our neighborhood, now, but its star was born in an early galaxy that doesn't exist any more; now that it's been discovered, Hypothesis B has been shattered.