Unlike the dogs in this study, who've all been untraumatized before they were acquired, the puppies I rescue are generally pitiful little things between 3 and 5 weeks old. They've already lived a tough life - starved, flea-ridden, often injured or sick.
I take them in, nurse them back to health, teach them basic manners, and find them new homes. Not one of the dogs I've rehomed has had any behavior problems as adult dogs.
The only dog that had issues as an adult is one I kept because she's a special needs dog, having suffered brain damage. And honestly, her issues have a lot more to do with her brain injury than any fact of having taken her from her dam too young. She's afraid of strangers until her brain cells kick in and remind her she does know this person - and that includes me. Even if she's napping on my lap or beside me, when she wakes, there's this brief time where she doesn't remember who I am and she'll growl or bark at me. But then you see her little brain kick in and recognition come up and she's happy to see me. She growls or barks at everyone the first time she sees them, and if she knows them, she suddenly pauses and she's suddenly happy to see them. She forgets where her food dish is, even though I keep it in exactly the same place all the time. When she finds it or I take her to it, she's happy. Overall, she's a pretty happy dog. And when she remembers something, that makes her even happier. It's like she knows she's forgotten something and when she remembers, it's not just that she's happy about whatever, she's ecstatic that she remembered. She does stress out if I take her somewhere new that she's never been to before, and stays stressed the first dozen or more times I take her there - until her memory kicks in finally and she recognizes the place. This is why I rarely take her to strange places unless I intend for her to go there often. I took her to the vet's office a dozen times before she ever saw the vet just to acclimate her, and I'll take her a couple of times before each visit to remind her she knows the place.
She's not destructive, or fearful on walks, or possessive of food. She doesn't play bite or chase her tail, or soil the house - she's very good about using her potty pads. She's no more attention seeking than any other dog. She's possessive of her toys when she's actively playing with them, but doesn't mind other dogs playing with them when she's not. She waits her turn for treats, even when she's lined up with stray dogs. She is reactive to noises but in a positive way - she alerts me to sounds I can't hear, making her a good companion to Itzl because he gets to go off duty when we get home because she is on duty inside the house.
I believe that training, especially early training, has a far greater bearing on whether the dog has behavioral issues as an adult than early removal from their dams.
I don't see where this study addressed training and conditions inside the home where the dog was taken.
Since I prefer tiny dogs to large ones, I know that many of them are poorly socialized and trained as puppies. Many people don't know how to train or care for a tiny dog, so they feed the insecurities and fears these little dogs have, making them hyper-aggressive, yapping bundles of furry neuroses - and it doesn't matter one whit how well cared for they were when still with their dams or how old they were before they were rehomed. These dog owners think it's funny or natural for tiny dogs to be trembling, yapping, biting bits of aggression.
Every Chihuahua and tiny dog I've rescued has been calm, well-behaved, polite little dogs, friendly to strangers and comfortable in strange places - except brain injured Xoco. Even she is calm, well-behaved, and polite, and as friendly and comfortable as it's possible for her to be.
The assistance dogs I train are all rescues. They may have been traumatized, they've certainly suffered abandonment and starvation; they're often sick or injured. Healed up, socialized, and trained, they become excellent service dogs. I check back occasionally to see if the dogs need retraining or up-training (even Itzl, as trained out the whazoo as he is, needs retraining and uptraining in new techniques and for new sounds or experiences), and help the handler/owner with that.
Regardless of their size or breed, all dogs need to be socialized and trained. I wonder what socialization and training the dogs in this study received.