According to TF1 he was arrested due to lack of cooperation with law enforcement regarding suspected complicity in drug trafficking, CSAM, and fraud. So my interpretation is that they repeatedly ignored french law enforcement asking Telegram to take down certain channels. Maybe warrant was the wrong word I am not a native speaker.
Not every threat model requires the security level GrapheneOS provides. My threat model ends with Google and other big corporation shouldn’t spy on me and if I lose my phone anyone finding it shouldn’t get in and be able to steal my identity. I think DivestOS and CalyxOS do a fine job with that.