It isn't rolled out to every channel yet. Back when the chapters feature was added to Youtube not every video would have it even if they had the correct syntax in their description. I happened to upload a video around that time and happened to put that syntax in my description and got it to work despite having a channel with almost 0 subscribers, making me think feature rollouts like these are essentially just random. Anecdotally, I suspect videos with these new features are also given an algorithm boost: My video where I had this feature randomly trigger early got over a million views despite that not being my intent and despite my channel being otherwise near non-existent in terms of activity.