No. It’s not zen experientially. For me, it’s not the brain being “empty,” it’s just [mostly] silent.
It’s that for much of what I read I go directly from “written words” → “meaning,” without having to go from “written words” → “their sounds” → “meaning.” Perhaps learning Japanese as a non-phonetic language as helped me. Kanji/Chinese characters are directly linked with symbolic and ideographic representations of the meanings, with only loose, temporal associations with the pronunciations. This is why advanced Mandarin and Cantonese speakers can read each others’ newspapers fine but not fully converse.
On what it’s like experientially, have you ever heard anyone trying to speak, such as with a brain injury… where they’re articulating every few words and mumbling in between and you’re doing your best to piece it together? For me, the inner monologue is a bit like that. Few syllables have sounds. Rarely is it “without any inner chattering” though.
While I’m not an expert on it, I think the most foundational aspect of it concerns numbers. When I read numbers, they rarely have sounds and, in fact, it can be hard for me to subvocalize them.
Another related topic concerns the voices of authors. Often when I read a book written by a living author, I go and find him or her on Youtube or elsewhere. I listen to their voice. I watch their mannerisms. When I do subvocalize, it helps me listen to the author’s voice in the subvocalization rather than my own. It helps me wrestle with not just the ideas, but the author’s ideas, in a more human context.
Anyhow, I used to be a prolific [English] reader, consuming a 300 or 400 page fairly technical book every week for a few decades. Somewhere, and with some intentionality, I lost the need to subvocalize. It’s just that for me, it came with some side effects. For instance, it’s now much harder for me to proofread what I write.
I’ve never really researched what the experiences are like for others. It’s just that words can now be with or without sounds. For me, it’s analogous to breathing. Autonomically, I seem to now read without much sound. Consciously, I can subvocalize with my own voice or the voice of the author. My brain picks one or the other and goes back and forth depending upon the nature of interest in the content.