You're right that the Philadelphia law is a law of general application. Does that make it exempt from the Contract Clause? If, in United States Trust Co., the state had instead passed a law that repealed all transportation laws, would the voiding of the contract have been valid? It still would have impaired the obligation of contract in question. As the Court said in that case "the Contract Clause limits otherwise legitimate exercises of state legislative authority, and the existence of an important public interest is not always sufficient to overcome that limitation."

Philadelphia's heart may be in the right place (although I think its motives are more related to the fact that the land is valuable for the first time in years and the city has a budget deficit) but its obligation to honor contracts is sacrosanct.