the kid didn't bite on the arm or neck of the mannequin. It bit the tit, which means the kid recognized the shape and thought 'food!' (there's a couple of other pics online where the kid literally pushed fabric off the tit of the mannequin on purpose, to get to the tit, so it's not like the kid is "exploring the world". It's about food. The same way (many) adults look at the outside of a McDonalnds building and think 'Food!' Or a better comparison would brobably be a plastic bottle for kids who used a bottle. It's just a shape they recognize and make a connection with food.

and yet, kids suck on plastic and random people and even pillows (saw that once)

it's not a bond with the mother specifically. It's a bond with the source of food.

(which is true for adults too, to have nice feeling for people who satisfy our needs)