The point is that the density of dark matter of the universe is not constant. There are regions depleted in dark matter (like around us) and regions with much more dark matter in it (our Milky Way has a halo of dark matter around it).

Over the course of billions of years, shouldn't the sun and some DM in its path have coalesced through gravity?

Yes, there are dark matter particles captured into the sun. That is the topic of the next article ;)