Right, the French authorities are going to present evidence that this dude was aware of specific illegal activity and refuse to comply with a legal warrant involving said actively, making him guilty of obstruction at best, and possibly conspiracy. Signal complies with warrants, they just don’t have anyone’s keys. Telegram has everyone’s keys, and theoretically could turn them over but they refuse. That’s a huge difference from a legal perspective.

This is precisely why opsec is more than just an app.

Leaky keyboards are a possibility, but what is actually far more likely is just that someone on the signal group chat was a mole who was archiving the traffic for the party. Signal has since made efforts to bring anonymous accounts to the platform, which will help thwart such attacks. Though against a state actor it is still not enough unless you take additional measures to obfuscate traffic. And then that still doesn’t protect you against some CCP brownshirt from tailing you and then snatching your phone out of your hand when you unlock it.