Factual error: Druig leads several warriors outside Tenochtitlan as it was sacked by the Spanish conquistadores, and they live peacefully in the nearby forest, for 500 years. The forest is of course the virgin Amazon forest, as captions say. Small problem; Tenochtitlan was in Central Mexico.
Plot hole: Deviants were created to get rid of dangerous local predators allowing intelligent life to thrive on the planets Arishem 'seeded'. He then created the Eternals to get rid of the Deviants once he realised they eat the species they should have protected. Problem is, it is stated that the Eternals go through their extermination routine multiple times. But the 'mistake' can't be happening all over again in a cycle, and Deviants would ruin a planet if left unattended.
Suggested correction: Arisham made them both part of the cycle to get the planet to be filled with intelligent life. So he always introduces Deviants before Eternals. It's a good way for him to tell the Eternals they are there to protect the intelligent life against the Deviants without knowing their true purpose. To keep them busy.
He refers to the introduction of Eternals as something he did "to correct my mistake." By that definition, he keeps making the same mistake over and over. If he just told them that they have to protect humanity and help their progress, he would obtain the same purpose. After all, for 500 years without Deviants the Eternals stayed put and passive as instructed and didn't create any particular trouble -if they did, they are a limited number he can easily pluck them out of the planet much more easily than a non-specified amount of ever-evolving beasts that he admits are out of his control and can grow much more powerful if they manage to kill Eternals.
He is lying. They are not a mistake, and they are not beyond his control. That's one of the main reasons why they go against Arishem in the first place, because everything is a lie, even after learning their true purpose. The Deviants are there to give the Eternals purpose, helping the intelligent life on the planet flourish, in the most natural way, for reasons we can only guess. The Deviant is upset too, learning the truth that they are only put on the planet for the Eternals to kill.
Suggested correction: It never says that the people who live with Druig in the Amazon in the present day are descendants of the people from Tenochtitlan. Nor does it ever say that the forest outside Tenochtitlan is the Amazon. He's probably been moving around for the last five centuries just as the other Eternals have.
Necrothesp
Never ever? He literally says "Do you remember this forest? Beautiful. It's the last place we all lived together. I've protected these people for 20 generations." They split after their argument during the sack of the town. If their base of operations exterminating the mutant space dogs in Mexico was in the Amazon forest, their logistic could use some work.
Sammo ★
Just because the last time they fought together was in Tenochtitlan doesn't mean that was the last time they lived together. They may have spent some time living peacefully in the Amazon before moving north to do their business in Tenochtitlan. And just because he's protected the people for twenty generations doesn't mean they're descendants of the people from Tenochtitlan. He may have found them later. We don't know every detail of the Eternals' history. You're just making assumptions.
Necrothesp
You are assuming the presence of a third party stranded for 500 years that the movie never showed before, different from the people that he led out of the city and that we have then to postulate he let go, in a location far off from the one of their last encounter. It's an assumption on entirely new details that you had to make up. My only assumption is to think that what is shown in the movie had purpose and fits, and someone just borked a caption.
Sammo ★
Who says they're stranded? He just said he had protected them for twenty generations. They'd probably always lived there. You're making the assumption that they must be the same people because nobody said they weren't. But nobody said they were either. Nobody in the film ever made a connection between the people in Tenochtitlan and the people in the Amazon. No mistake has therefore been made in either the dialogue or the captions.
Necrothesp
I noticed the same problem, the scene indicates the location as "Amazon" (it could be any of the Spanish speaking countries that have part of this forest), but then, Druig comes with the affirmation you pointed. It's obviously a geographical inaccuracy.
They don't speak Spanish in the Amazons.