[drzen]

You need to read all of his advice.

He's saying that if on the flop there are two big draws (straight and flush in your example), and four people call, it's likely that both draws are being chased. The outs for both together make you a significant dog (if it's both the low cards that are diamonds, they have 15 outs; if it the 9 is a diamond, they have 16). Worse still, even if you bet the pot, you will sometimes have another caller, with OCs, making as many as 22 outs against your hand.

Then he's saying that betting out will not make them fold, so a bet is not protecting your hand. Each will be wrong to call, and you have more equity than either one, but you are a dog to both together. It's called "schooling". You beat each fish, but you cannot beat the school of them.

I'm guessing you haven't played much limit. If you had, you'd be used to this sort of concept. You'd check, let the maniac bet, and put in a raise to fold out other players. If maniac's not in LP, you could just hope button will bet if it's checked to him. But your pro is right, I think. Betting out will never protect your hand against a fishy table on this kind of flop.