If the book is recorded at essentially half-speed, then yes fair point. :)

> listening at 2x taking twice the brain processing feels off to me

It's not about the listening -- you're right, that's all habitual in terms of converting sounds into words. It's about the cognitive processing of the content. Whether you're listening to fiction or non-fiction, you're engaging in twice as much imagination, world-building, conceptualization of what you're listening to in the same amount of time... therefore a much greater proportion of your total attention and focus is engaged in that, which leaves less for paying attention to the world around you. It's mathematically twice as much information streaming in, and our brain's attention mechanism is finite.

People aren't usually consciously aware of these things, in the ways they insist they can drive fine even though they're tired, even though they've had a few, etc. Because we literally can't notice what we're not noticing. But accident rates tell a different story.