The guaranteed lucky trades are a catchup mechanic for people that were playing when the game first came out, left the game, and are just coming back. It allows those players to receive up to 10 good pokemon that they may have missed out on.
If a player has received less than 10 lucky trades, and that player trades a pokemon from July or August of 2016, that trade is guaranteed to be lucky. It doesn't matter how many lucky trades the other player has received or the catch date of the pokemon they are trading. The player trading a Jul/Aug 2016 pokemon must be the player with less than 10 lucky trades.