Is it possible that she would exhibit this behavior in the wild? I don't think so. She would have her mother's teats available AND she would be clinging to her mother's fur. Non-human primate mothers don't hold their babies - their babies hold onto them.
Lolita too has exhibited occasional tailsucking, even though she's been bottlefed since she arrived. (Spider monkeys don't have thumbs on their hands, and I have never seen her suck the thumbs on her feet.) She was very envious of our grandson's pacifier, which he generously offered her, but she was disappointed and dropped it almost immediately. I've since given her pacifiers of her own, which have also been ignored.
Venecia, another young howler monkey, was with her mother longer than Marisol and was weaned when we got her...or at least refused goat milk and formula. I think it's significant that I haven't seen her sucking on her thumb or fingers.