The keyword here is "Primary"; in a Primary election, you are running against one or more candidates from your own party, and if you win, your name goes onto the General ballot.
Stealth Republican. I can't say this for sure but, since the district leans so far left that the Republicans didn't even run a candidate last time, this individual probably just ran as a Democrat even though she intended to support Republican positions once elected. Campaign as if you have the same ideals as the voters, then turn coat once you're sworn in. It's been done before.