[6.0 - 6.4 Spoilers] An interesting video detailing Venat's polarizing reception

[Absimiliard]

I don't think their point was that they were intact, but that sundered people weren't broken to a point they couldn't form coherent thoughts.

The fact Emet couldn't understand them would imply the Echo wasn't working, but on the other end, they would have lost their memories after their deaths.
I see a few possible explanations:
- The cutscene is wrong and Emet-Selch meant they couldn't form words but still understood them with the echo ;
- There was a time of adaptation where the sundered people were so disoriented they couldn't form coherent thoughts ;
- These memories took a few rebirth to be cleansed, somehow ;
- Zenos isn't the only one who dreams of the Final Days, perhaps it was more common back then.

Pure conjecture though,

I can see where your interpretation might come from. However, the rest is a fair bit less vague. It makes clears some of the things lost in the sundering. The loss of culture and knowledge implies an incredible level of corporeal and spiritual damage, as does the loss of language as a whole. These are things intrinsically tied not only to civilizations but to individuals. To lose one or any of them is to lose an integral piece of oneself, and to lose them all is to be lost in turn.




To note, all of these little cutscenes are accompanied by narration from a neutral party. NieR does not make use of the unreliable narrator method.

As to the Echo not working; that appears to be correct, although it is important to note this would not be a problem on the Unsundered's part. Failure to communicate would instead have to come from the early sundered being so utterly ruined as to render even communication via the soul an impossibility.