Planes back then regularly had to land to refuel.
And if you ever get involved with old learning how to fly/ppl and dealing with old old CFI's/ex military, they love to tell you of their old glory days, and if you question they'll drill down exactly how it was.
So I'd say it was probably, and further proof, just look at frank abangale. So easy a teen did it.
I've travelled in many places where I don't speak the language and managed just fine with a little creativity.
"Act Three" here: https://www.thisamericanlife.org/477/getting-away-with-it
So, I dunno, I can believe it, although I don't know exactly how connections worked then or if it could have been the same plane stopping in LA and continuing to Japan or what. But in general, yeah, "security" wasn't actually a thing on planes then. There was no security checkpoint at all to get to a gate before the 70s; you didn't need a ticket or to show ID to go through the security checkpoint that did exist before 2001. It's easy to forget or not realize how much thing have changed.
Or, it's just one of those rumors that people like to adopt and claim as a story that happened to htem, when really they heard it from someone else that claimed it happened to them, etc. That also happens.
It was like a ship, friends could accompany you and then leave when they asked for all guests without a ticket to depart. I was just a kid but I do seem to remember them checking tickets. But I suppose you could wander aboard and find a good hiding place, unlikely but not impossible.