Hello, Hello
full wiki statement:
The most common Ambassador dilemma early on is drawing it with 3 Copper + 1 Estate. Contrary to popular practice, Ambassadoring 2 Coppers (rather than 1 Estate) is the best play; deck-thinning is, at this point, more important than a marginal increase in your attack. Moreover, Ambassadoring the Coppers decreases the chance your opponent will Ambassador you two Estates while increasing your own chances for the same.
I read this as I would get 2 Estates. But is the intent of the statement that the opponent will return 2 Estates to the Supply so that you would get 1 Estate?
Note that the wiki statement is about tactical advice, not about rules. The rules are you can return 0,1, or 2 of the same name cards to the supply. Afterwards opponents get 1 copy of a card by that name from the supply.
What the wiki advice is about is: if you can choose between returning 2 coppers or 1 estate you should choose to return 2 coppers. The line you quoted simply says that if your opponent gets more copper in his deck he is less likely to get a hand with amb+2 estates and that you are more likely to get this hand because you have less copper and your deck is thinner than when you would've returned only one card.