Why not? It's just a shorter way to say the name.
The plural of the proper noun Philly is Phillies. (As a nickname for Philadelphia, it has no plural.) The plural of the common noun filly (young female horse) is fillies.
Philadelphia dip is made in Italy. I don't get why it's called philadelphia!!
It is Philly when you are referring to Philadelphia; and when talking about a female baby horse it is filly.
"Philly" is short for "Philadelphia." There is also a team here, named the Phillies.
the philadelphia athletics
philly means philadelphia