The initial investigation which triggered the arrest was made by the OFMIN ("Office spécialisé dans la lutte contre les violences faites aux mineurs" basically the government branch tracking and fighting CSAM).

Supposedly, Telegram (and by definition of the french law, the CEO) did not respond to requests for takedown of harmful content (or not enough or faster?) from the the OFMIN. This triggered another investigation looking globally at how Telegram handle content moderation on the public part of Telegram (Channel) which lead to all others charges of complicity.

This is basically the CEO taking the fall because the (unreachable by french law) Telegram company is not on french soil and he made the mistake on landing here.