For me it depends greatly on the dog. Some dogs really do need intensive well thought out socialization early if they show signs of stranger-danger behaviors (avoidance, barking, hackles). What you do during that critical window you have with a new pup could make a difference between a safe in public, aloof dog, and a nervy, barking mess.
Ideally, you get to start with a good pup with stellar nerves, but that isn't always true, despite the most well thought out breeding and best of intentions. That is where really well done socialization and exposure can and does matter the most.
There is a lot of merit in molding behaviors you want at a young age.
I find this article much more useful, because it goes into details of what to do and how to do it - Socializing the Schutzhund Puppy | SCHUTZHUND LIFE
From the article:
"While we go about socialization, we must also remember that every puppy is different in its temperament, drive, personality, strengths, and weaknesses, with genetics playing a huge role in determining all of these. Thus, how we accomplish this goal above will vary based on each dog; there is no "one size fits all" approach. Each handler must do what's best for their particular dog, taking into account their puppy's particular temperament, strengths, and weaknesses."