I don't think Monuments had anything to do with your victory here. The lack of 11 VP Tokens wouldn't have made any difference and the handsize reduction of Monument only helped you once, in the final turn. It's more that you got one great turn with completely similar decks while he didn't.
While Horse Traders looks good for discarding cards, Festivals' lack of drawing is bad here, since something like Festival-Festival-Horse Traders-Watchtower doesn't work. I still want Horse Traders here for the ability to get me to 5$ more often, and for the discard in more crazy megaturns.
I don't know why nobody bought a single pawn here, or tried to set up a megaturn with the help of watchtower topdecking cards. The first Festival was bought on turn 10, which I don't understand either.
I tried the setup out
here. I only played the first try where I hit 5 at least a single time during the second reshuffle. I accidently bought a sixth Festival, but this shouldn't change things too much (I had two Festivals anyway next turn before I played the first Watchtower - EDIT: Well, it did matter, providing me the 8th buy. So instead, I could had "only" bought 7 Provinces on turn 15, unless I would have grabbed a Pawn or so).
I am only 1$ short of buying all 8 Provinces on turn 15, so I had to buy 8 Duchies instead, which makes it a bit less impressive, but hey. The game would have ended on piles at this moment.
I hope the log illustrates what I mean.