Factual error: Aquaman lifts a submerged submarine towards the surface, which should start to sink again straight after releasing it, as its ballast tanks are still full of water. It shouldn't stay floating like that.
Factual error: An M203 round arms at between 14 and 27 meters, and produces a ground burst that causes casualties within a 130-meter radius, and has a kill radius of 5 meters. Even if it did work all Manta and his father would have been killed. (00:18:35)
Factual error: Atlantis was in the Sahara when it was still an ocean, that was about 4000-8000 years. But part of the riddle was to find the king, Romulus. Rome was founded around 750 BC. So no way for him to know where they would build a statue about someone that wasn't born yet.
Factual error: When Jesse shoots the M203 round at Aquaman, the distance is too short for it to arm and explode. (00:19:40)
Answer: You said it yourself some might have stumbled upon it, others could have used the map, which is why it was hidden in the first place.
How could they use the map when Arthur was the first to find it...quite frankly to stumble on it is impossible, you would have to randomly travel to the trench, go through the entire kingdom, see a tornado of death and decide to go in, get spat out and not eaten, get to shore, check behind a waterfall and follow it down...bit much?