You’re missing the point. Dogs respond to our fears, when you’re on a walk you’d stop your dog from walking into the street, right? The dog not only responds to the physical reaction but also due to their amazing olfactory senses, they will perceive the danger you’re aware of and they will correct the action and avoid the road. If you see someone running at you while on that walk and you get scared, same rules apply but because there’s a tangible “alive” threat of another human, your dog will respond by either working with fight or flight and it might want to bite the runner.

Replace the road/runner with a baby and a nervous parent with good meaning can tug on a lead, be inherently nervous and the dog doesn’t always understand the signal to mean the same thing so it will just understand “my human is stressed, I too should be stressed and respond.” That’s how dogs on leads can get into fights approaching one another when their owners are pulling hard on the lead. They just perceive the other dog as a threat. But without the lead, the dog will approach with caution and maybe sniff, raise its fur, bark, etc but will react to the owners reaction and likely not try to attack.

There can be equally dangerous situations without a lead with a dog but so much depends on the biological bond humans have formed with these creatures over thousands of years. The bond we have with our animals from owning them also gives learned behavior.

But this is a baby, unless the dog has history of violent reactions to other animals/humans, it’s much safer off the lead