I'm pretty sure that he's talking about opponents who he knows will play a certain way against him in a certain situation. For the three-flush, he knew more or less exactly how the opponent would play against him if the opponent had a flush, if he didn't, etc. so he can confidently bet and extract as much as he can out of the other guy's top pair. Of course, the perfect opponent for this type of situation is the one that bets hard on the flop and turn with a flush draw (or anything else that can't beat a two pair), and then you can shove all your chips in on the turn for either free money or a very nice favorite if he calls.

The flush draw example is a good example because it's probably the easiest to spot(it was for me), but it's not as easy to do because you're shoving all your chips in when he could have just been value betting with trips to try to drive you out of a flush draw. This means that you need to be able to spot that he plays a made flush/trips in a very predictable fashion, while also playing flush draws in a very predictable fashion. These types of players are more common than you may think, but it takes a keen sense of observation to first figure out that the opponent plays it that way, and then a good deal of waiting getting into a three-flush situation with 2-pair or better yourself.

Also, be sure to realize that extracting the most money with 2-pair is only half of it. The other half of why this situation is so perfect against the perfect opponent is that you know when your 2-pair is beaten. You lose the least and win the most, that's how you win at poker.