Hardly. The DCEU Wonder Woman is a character surrounded by myth, legend and hyperbole. Unable to be killed? Not even close to the truth. Ares is an unreliable narrator.
When modern Wonder Woman in Batman v. Superman faced Doomsday, it was clear she was outmatched. Without the intervention of Superman and Batman's Kryptonite spear, alone she would have simply been beaten to death. Doomsday just kept getting more powerful with each injury she caused.
Diana is amazing. She is formidable, but it is clear she can be beaten by an enemy capable of dealing out more damage, outfighting her, or using an attack she simply cannot overcome, which Doomsday seemed quite capable of doing if he had been given time.
You have to look at the gods the way they see themselves. Before the arrival of aliens on Earth, the Greek gods were the most powerful beings they knew. Assuming there were no other gods, from their perspective, only other gods or monsters from their mythology had the magical or physical capacity to defeat them. From their perspective, at least until other aliens arrived, "only a god could defeat a god."