This is goofy because Philly's population has doubled since the 1960s (which you're well aware of), why would you ignore that fact just because the article did too?
Because population size isn't the end all be all with interpreting crime numbers. It isn't a matter of, well, the population has doubled so the number of murders should also be multiplied by two.
Police force has increased too, methods of policing have advanced, the ability to monitor citizens has advanced, ways to reach out to citizens has advanced, more time has gone by to put in place good preventive programs, etc.
Moreover, other cities have done better in bringing down murders and, as I stated, his immediate predecessor averaged nearly a 100 murders per year less than him, which has nothing to do with population size.
Why are you ignoring context you can figure out for yourself and pretending that the article in the OP actually had a valid point when it clearly didn't?