It's not, Facebook processes the photos on their end and now also apparently feeds it to generative models to offer something (don't remember what exactly, managed to avoid this). At least that's what some options said last time I went exploring the app's settings page.
Well then the next one might be. What I'm saying is that, per my recent memory, the options in Facebook settings menu around camera roll suggestions don't, in any way, even suggest the processing might be happening on-device only.