I will never understand why people defend this idiot. Plenty of secure chat apps exists in the EU without problems. Matrix for example is used by both the German and the French government. Telegrams CEO is just reaping what he sowed by ignoring French warrants.
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.