Yup, that's good advice - labs are notoriously destructive chewers as pups - but they DO grow out of it. Mine stopped completely around the age of one and a half years.
With all dogs, it's consistency that makes them understand allowed and not allowed, so you might have to really devote yourself to working with the dog and watching his every move for a while. Find something he likes to chew, toy wise, and have a TON of it for him. Mine liked a nylabone and a Kong that I filled the gaps in it with peanut butter - kept him at it for hours. But, don't have a great variety of things he can play with - it'll confuse him as you teach him what's allowed and what isn't.
When he's chewing on something that's ok, sit with him, pet him, play with him...make him feel like he's doing something good. When he moves to something he's not allowed to chew, scold him (anything more than a shout or a tug away from it is counterproductive). Show him the "good toy", offer it to him or better yet toss it to get his attention as an alternative. When he starts playing with it and chewing it, praise him generously. Do this again and again for a few days. You really have to reinforce two behaviors - clearly and without exception what IS allowed, and what isn't, and that when he chews his toys he's going to be happy and when he chews something bad, it gets him nowhere.
Not saying this is easy, you're fighting his nature, but if you work at it a little and be patient with him, you can correct the behavior. It's always better to reward the right behavior than to punish the wrong.