Stupidity: Arishem's plan with the Deviants hardly makes any sense; he sent out biological beasts that he can't control, to kill off the natural predators of every planet. Forgetting the fact that obviously it didn't work (but the movie does not say that) because Earth has always had predators, that's a terrible plan to begin with, since any ecosystem needs predators or the other animals will grow uncontrollably.
Stupidity: In the scene around Tenochtitlan, the Eternals have a device that allow them to locate the Deviants - they know for sure that they have exterminated every breathing one. In the rest of the movie this technology is never brought up again. It also makes it really odd that it took them 6,500 years to kill the Deviants, apparently never actively hunting them.
Stupidity: When Thena suffers from her first spell of Mahd WY'ry, Ajak knows what it is and flat out tells her that they have the technology on the ship to wipe her memory off to start over. Somehow none of them asks themselves any question about any of this. They all think that this mission was their first mission, but for the plot to work everyone in those boring 6500 years together must have never talked to each other about their past - Sersi accepts instantly the fact that her home planet doesn't exist, so she doesn't seem to have any strong recollection about it. And nobody ever inquires Thena about those memories of other planets ending up in disasters.
Stupidity: Phastus' partner learns with astonishment about the giant-shaped rock in the Indian Ocean from the TV and asks "You guys did that?" He must have lived under a rock to be asking that in a scene that takes place two weeks after the events and without a doubt has been all over the news ever since. (02:16:45)
Stupidity: The Eternals are in a rush to figure out what is happening to the world and reunite the group. So naturally they go to their spaceship, which also happens to be where the speedster of their group is, as last part of their trip. Ikaris (which admittedly has an agenda of his own, but the others don't know it and know his abilities) is with them from the beginning, can easily fly across continents and even makes it to the Sun from Earth in a comically short time at the end of the movie, but instead uses their comparatively slow spaceship to spread the word.
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.