I think this is the answer. I think it's likely we're all experiencing something similar, but we're all describing it in different ways. People in the article who seem shocked at the idea of an internal monologue are getting hung up on this assumption that people are hearing an audible voice commenting on their every move, but I don't think anyone is actually experiencing that unless they're schizophrenic.
Some people reading this will say "no, I really do hear a voice!" But I'd ask if the audio of that voice is capable of drowning out the noise around you. I'm guessing it isn't. And if that's the case, we've discovered we're both working with different definitions of what it means to hear.
I think the more likely explanation is that many of us have developed a habit where we activate this pseudo-voice in order to have pretend conversations in our head, and others just didn't develop that same habit -- although they could probably do it if they tried.