That’s what Buttigieg indicated. There are secure means of communicating classified information, with a process for ensuring people with clearance are the only ones allowed to view this information. I imagine those rules are in place exactly for this reason, to prevent accidental leaks.
The rookies that Trump hired probably didn’t see the harm in doing it in this way, I mean come on, Signal is secure so what is there to worry about?
Who knows what else has been leaked to the wrong people accidentally.
You also clearly implied you like and support Nazis. No I won’t provide a quote. But it was clearly implied.