Just declaring anyone with a rifle as part of the enemy is a wild way to get yourself in front of the Hague.
But also no. It depends on what the ambulance is doing. Going to the hospital? That's a war crime. Moving armed soldiers to the Frontline, that's a target.
It's not nearly as simple as gun=target. This is the military, not the American police.
If a combatant is hors de combat, no longer able to fight, then they are no longer a valid military objective.
If the IDF shuttles soldiers ready to fight to the front using an ambulance, it’s a war crime. Shuttling injured soldiers from the front to the back is not a war crime.
Hezbollah and also Palestinian militant groups have been observed to use ambulances to do the former.
An injured fighter is no longer a combatant and entitled to medical treatment like any other human being.
But by blowing up the ambulance so that only body parts can be found after, it can always be claimed that the patients were still healthy before being bombed.
Specific ambulances are only attacked if there’s intelligence like surveillance that says it’s being used to transport combatants or weapons. If all ambulances were systematically targeted, all ambulances would camouflage as civilian vehicles.
You are correct that persons hors de combat are protected.
Evidence in war zones is always difficult, especially when combatants don’t wear uniforms. You can easily make a combatant look like a civilian, by removing the weapons from the scene. Making civilians look like combatants just takes putting a weapon next to their body. Independent neutral investigators will rarely arrive at a scene before one of the belligerent forces.
The Guardian are pretty much the voice of New Labour, who are totally in bed with Israel (they're still sending them weapons and even sent surveillance planes to help them in Gaza).
If The Guardian is actually critical of Israel and the IDF that's a pretty good indication that the Zionist Genocide has already burned most of their good will even in Britain which is one of the most right wing states in Europe and has a history of invariably either being one or supporting White Colonist States in their Genocides of the locals (remember how they supported Appartheid until the last minute and even called Mandela "a terrorist"?!).