after hackers hijacked a user account and gained access to public chat rooms.

The platform wasnt breached, a single account was taken over…

But then they write this:

While French officials described the breach as limited in scope, an alleged hacker claimed to have obtained 73,467 user accounts, 643,459 messages, 876 chat rooms with message history, 59,386 media files totaling 13.51 GB, and references to documents marked “Diffusion Restreinte,” a French government restricted-distribution classification. Access was allegedly gained through social engineering of an account associated with Tchap’s education environment.

So who knows, but i doubt they were able to take over that many accounts. That would mean there was actually a technical failure of the underlying auth or matrix stack.