Plot hole?

Could be a plot hole.

The whole dreaming/sleeping in the water with the helmet on in the pod confused me. I thought those were made up dreams
from those CIA guys at first, but then I realized that they were relived memories of Eleven trying to help her to remember her powers.

Yeah, it feels like a re-write, but she definitely knew how to talk. Her silence was more a coping mechanism for the trauma she went through, not a lack of ability - plus, she wasn't familiar with the outside world and didn't know the words for a lot things she witnessed and experienced after escaping Hawkins Lab.