Garvey is only at the top because there are three Democrats in the race who are splitting the Democratic vote. The Democrat will easily win the runoff.
The article doesn't provide the poll and doesn't reference any numbers, just three names. Schiff, Porter, and Garvey. Seeing as how Garvey is the only Republican, and how blue California is, it is more likely that the poll shows Garvey in top because there is still a primary and the polls are split on the Democrat side.