Cellulose in cardboard can still be broken down by bacteria (and other things) as it's the same as the cellulose in your food scraps, it just takes longer as it's so concentrated and tough. So like other foodstuffs, it gets eaten along with the bacteria that populates it.

BTW, while worms might not eat the food scraps specifically, that doesn't mean they are not ingesting pieces of rotten veggies or fruit while they are trying to get at the bacteria. It's more of a note of why the worms will wait for a while before attacking your food scraps. With the cardboard, you can sometimes see it in fibrous looking castings.

Cardboard decomposes readily. The starch used to bond corrugated cardboard may also make the worms happy.