1) If someone on the street approaches you, they are probably up to something.
2) If you randomly select someone on the street you have probably picked a moral and upstanding person who would love to do you a good turn.
Because of (1) people tend to be a lot more defensive than they need to be when assessing what the average stranger is likely to do.
You can add a few more layers like seeking out someone in a uniform or who has a stroller, but generally speaking most people minding their own business are perfectly safe.
Honest grown-ups will be annoyed that a lost child refuses to come with them to the obvious "lost children" place, but they'll put up with it while somebody tries to figure out where the people responsible for that child are. Somebody with nefarious motives will probably need to take the child somewhere else though - by refusing to go they're protected.
tl;dr for the rest: it means: "Never go anywhere unless it was planned beforehand".
My son was always hyper social. (I have no idea where he got that from.) Would walk up to any one, everyone and start a convo.
While it's fantastic that my son was so open, trusting, there is still a risk. A little girl two grades ahead of him was abducted and then found dead on a beach. (Oak Harbor WA, should any one want to dox me.)
I am nervous with one other person in my subway car.
I feel completely safe with 5+ unrelated people in my subway car.
When I moved to New Zealand it was the opposite. There is more interaction with "regular" people. They are helping me or asking for help.