I was wrong here. In multiplayer games you're more likely to get a higher number of curses, even if you get a curser yourself. Heck, you can get above 10 curses if the other players have Moat or Trader or something. However your opponents would also curse each other, decreasing both of their chances of beating the gardens player. The curse slogs tend to be more pronounced in multiplayer games (since the average number of curses per player increases) and that tends to strengthen alt-vp strategies.
No, you were right. The only possibility that you will receive >10 Curses is if Moat, Lighthouse, or Trader blocks incoming Curses for opponents or Young Witch plays get blocked by banes. Otherwise, you'll more likely receive ~7 Curses in a 3-player game if all players try to give Curses.
You do get them in your deck faster, though.
This is true if you get a curser yourself, which wouldn't see many plays if you're going for a Gardens rush. In a 3 or 4 player game, having your opponents play 10 witches combined leaves you with 10 curses and the other 10/20 split amongst themselves. If you got a curser, you'd have less curses, but then if you got a curser in a 2 player game, you'd have even fewer curses (~5).
Regardless, the lone gardens rushing player benefits in curse games as the number of players increases. Not because the gardens player gets fewer curses, but because the other players have more curses slowing them down. The same is true for other alt vp cards like Silk Roads and Fairgrounds. The multiplayer slog is a side of Dominion that we miss out on by playing mostly 2v2's.