It probably is related to DMN, the less active the DMN is the less "random" mind-chattery thoughts happen. Meditation is shown to decrease DMN activity over-time, which would decrease the occurrence of these thoughts
Anecdotally there was that anon who went from only inner voice thinking to developing unsymbolized thinking after taking on mindfulness meditation-https://boards.fireden.net/sci/thread/12050903/
And in my case I was similar to OP with no inner voice thinking, but later it developed but so did my distractability and mind-wandering tendency. Mind-wandering in the over-active DMN style is different from the one I used to have which was more convergent thinking and idle thoughts but without inner voice accompanying them or random monologuing in the mind, before I would only utilize inner voice when recalling song lyrics or reading something from a book in my head, but never to THINK, because that was too slow and inefficient and I only just now realize it.
I think a good study would be to take long-term meditators and determine if they use inner-voice thinking or not and measure the activity in the DMN when idle (when its supposed to be active) so basically corroborate the reduced DMN activity with reduced inner voice thinking and bam, correlation established
That would be a useful study but I don't think anyone has done that, studies on meta-cognition like that are nearly non-existent besides that Hurlburt study on unsymbolized thinking