It continues next tweet with 'and Detroit'. He's continuing down this dog-whistle line (what do Philly and Detroit have in common?).
However, over 100 million mail-in ballots in cities (like Philadelphia and Detroit - this is presumably a clarification for people who don't know what cities are) is not impossible - most people live in cities, and a lot of people voted by mail.
ETA: Just checking the maths, he's specifically name-checking PA and MI states. That's 36 EC votes between them. He would still need to take Arizona, but if he does that, he can win by getting PA and MI reversed. (Just trying to work out exactly how it is he thinks he can still win. If Arizona turns red, Trump might just keep on down this hole.)