The article spoke of exactly that - the entire study was based on anecdotal information, since there is no way to know for sure what is actually going on inside

The article spoke of exactly that - the entire study was based on anecdotal information, since there is no way to know for sure what is actually going on inside another person's head. Their only solid point was that the voice in our head reads to us and talks to us in a similar fashion to what a child receives from adults during their formative years. But they even admit to not being able to tell even how often this is the case. Interesting, but admittedly weak evidence, essentially. It seems there are a lot of studies out there on why we have an internal voice, why we have it, etc.