My understanding is that that is true for immigration, but not for customs. There are still limits on the amounts that can be imported into Switzerland, even from the EU, without duties [1]. Some products are more restricted, e.g. a limit of 0.5 kg of meat per person per day [2]. Anecdotally, this is rarely enforced, except during summer barbecue season, and customs officials have the authority to stop vehicles within 10 km of the border.
That's the point. They do check quite a lot of people for undeclared goods. However they're no after those who buy stuff for themselves, but for example restaurant owners who try to smuggle in cheap meat in big quantities.
The Schengen agreement does not include customs. That's in the European Union Customs Union, which Switzerland is not part of. You still have to self declare goods when entering Switzerland. Border patrol checks on suspicion.
