The bombing is worse, but using signal instead of official communication channels is still really fucking serious. They want to plan and commit war crimes and avoid any responsibility for it by trying to keep it from ever getting under public scrutiny.
Especially since this is what was caught. Considering how brazen it is, and how those involved are trying to worm out if it, you have to wonder if there's similar or worse going on that flew under the radar because a journalist wasn't accidentally roped in.