If I had to guess, "Lean in" would mean people watch the shows because they're genuinely invested, while "Lean back" would be putting on a show to have in the background while you do something else, or not really caring about what you're watching as you relax and chill. Like, you lean in towards the screen, versus leaning back on the couch to get comfy.
I think that's exactly what they're saying lol. To be fair, I do know a fair amount of women who like to watch any true crime thing. But that's about it, if anything, I'd say women tend to get into "fandoms" more.
