Pellets aren't hard enough to wear down the molars. A pig full from pellets is not going to go to a lot of effort of power chewing on hay to fill the stomach. A pig whose stomach is filled only by hay gets one heck of a workout on the molars and the jaw muscles preventing TMJ in the future.
Definitely pigs can overeat - obese pigs do not do well with sedation for major surgery such as spays. And if they don't get enough exercise they get fat just like humans. Overeating is not a big problem for them but their systems were expressly designed to live on a diet of low nutrition forage grasses that forces them to graze continuously to get the nutrition they need to maintain. The continually pooping is proof of the special way they are designed. The food moves through quickly making room for more. The eating of the special poops is more proof. They eat them to wring the last bit on nutrition from the grasses and hay and to get vitamin B.
The greatest risk a pig faces from not eating enough hay is malocclusion caused by TMJ.
Chewing on a wood block is pointless. This will only control the incisors unless the pig grinds the wood with molars. This is a hamster thing - not a pig thing. It's the molars that usually control the incisors - If they overgrow they prop the mouth open allowing the incisors to overgrow to make contact.
Pellets should be treated as an augmentation to a primary hay diet.