Philadelphia uses a variety of symbols on its streets to indicate bike-something. That something can mean BIKE LANE (a "bike lane") or it can just mean REMEMBER THERE ARE SUCH THINGS AS BIKES (a "sharrow;" here is our primer on sharrows).
Unfortunately, these symbols are, in the grand scheme of what you can do with paint, pretty similar-looking. And for better or worse, we've got 'em in Philly. This can lead to confrontations between people in cars and people on bikes when somebody believes a sharrow to indicate a bike lane.
So here is a reminder of what these symbols look like, and what they mean.
Sharrow ("shared-use arrow")
15th Street through Center City has two lanes, and contains sharrows. The bicyclist above is perfectly correct. That sharrow does not mean bicycles have to be in the left lane. It is just a reminder to drivers to watch for bicyclists. Both cars and drivers can and should use both lanes.
Bike Lane ("a lane for bicycles")
Pine Street through Center City has one vehicular travel lane and one bike lane. The solid line (in this case two lines, because it's a buffered bike lane), combined with the different-looking bicycle icon and the directional arrow, indicate a bike lane. This lane is for the use of bicycles only. Bicyclists can ride in the vehicular lane in order to, for example, avoid debris or a blocked bike lane, or to make a left-hand turn.