Yes it is true that the Federalist papers were written by those within parties, but the papers were not actually written by only Federalists. James Madison who wrote Fed. 10 was actually a Democratic-Republican, which differs from Federalists in many ways. The argument made using Fed. 10 was that factions/parties are inevitable in places where liberty exists. Liberty allows different thoughts to occur and different opinions to voice their opinions which is what this Fed. paper argues for.