To make any progress on this question at all, we need first to come up with some definition of internal monologue. Even if we may need to modify it later, there has to be a starting point.
Otherwise, nothing can be established at all, because for any statement there always will be someone's understanding of "internal monologue" for which the statement is true, and someone's else understanding for which the statement is false...
I'm sure inner monologue just cashes out into the ability to reflect on your own thoughts. And for one to say that they're not having that experience also involves a claim about what they think other people are having which would make me doubly skeptical.
In practice, when you see people arguing about whether they have an "inner monologue" or can "mentally picture objects" on social media, it's more of a contest of who is the most unique in the world rather than anything that sheds clarity on our subjective experience.
Otherwise, nothing can be established at all, because for any statement there always will be someone's understanding of "internal monologue" for which the statement is true, and someone's else understanding for which the statement is false...