I worked at Meta. Yea, there was a promoted culture of flatness, but that only applied to the IC side. Within engineering there are separate levels but everyone has the same title "Software engineer" so that (supposedly) level shouldn't be a factor in whose argument is correct. In practice, everyone knows who's high-vs-low level, so it's mostly a charade.
On the management side, there are clear titles, reporting hierarchies, and areas of ownership. They decide what gets prioritized, staffed, and shipped. As an IC, you can make technical decisions and recommend product ideas, but you'll never ship functionality to users without PM, design, and eng leadership approval.
Within RL, like everywhere else at Meta, there are VPs and Directors whose job was to set strategy and create roadmaps. RL had a multi-year roadmap aggregating roadmaps from all the sub-orgs within RL. There's no world where a single IC, even at Carmack's level, is going to subvert that hierarchy. The job of a senior IC is to influence and check their management counterparts to make sure they're prioritizing the right things. Clearly he wasn't able to do that to the extent he wanted.
When I was there a couple years ago, I looked into joining RL a couple times but got advice from coworkers that the rotating carousel of leadership led to changing roadmaps/priorities, so it was a thrashy place to be.