What is the difference between these two usage of 'could' ?

In an article from British Council, it says as below:

"We use could and couldn't to talk about the past:

She could speak several languages."

Link: https://learnenglish.britishcouncil.org/english...

But when I say like this, people consider it ungrammatical:
I could get up early yesterday. (and I did)
Because it should be 'I was able to'.

Related question: https://hinative.com/en-US/questions/16874539

What' the difference between those two?

P.s. There's another sentence of 'could' indicating the past. This is from a dictionary: She could read when she was four.

I think of COULD as "being able to"... not necessarily something in the past.
"She could read when she was 4" means: She was ABLE TO read, and yes it's in the past.

BUT here's an example of future use:
"What do you want to do tomorrow? We COULD go swimming, we could ride our bikes, or we could take a walk".

Your example of getting up early can work too... but I think you need the other part:
"I could get up early yesterday, SO I DID". that sounds OK to me.

the languages example, I guess it's the past too:
she COULD speak several languages... so that's why she TRAVELLED alot.

(it seems to need the other part of the sentence)

hard concepts... !