That’s actually a really good question and I don’t know the answer. My best guess is that the founders didn’t intend that as a purpose of the EC, that it’s a secondary effect that modern political scientists have theorized.
You’re right that Washington pointed out the dangers of political parties, but that was in his Farewell Address which came at the end of his second term, by which time the Republican and Federalist factions were already becoming baked in and he had experience with Jefferson’s and Hamilton’s fighting within his administration. I don’t know that he or other framers were thinking of that when they designed the EC during the framing of the constitution eight-ish years prior.
For all I know, however, there might be a Federalist Paper that lays out partisan moderation as a function of the Electoral College. Maybe someone with more expertise can correct me here.