What the parent post was saying was well described by his examples of where the article is obviously superfluous and doesn’t provide contextual information - which is the majority of their uses. There are situations where articles carry information (nobody disputes that I think), but they are used far more frequently as grammatical filler.
“I saw that good one” -- translated to Croatian, this wording strongly implies that both knew upfront which movie was good and he watched only that one. But according to the parent's explanation it may not be the case, Alice has only learned that one of the movies was good.
and
"The one I saw was good" (he picked a random movie from the collection and it happened to be good -- which would be "a good one" from the parent's example).
> Bob: I saw the good one.
Weird. Before I read your explanation I interpreted this as Alice and Bob having a prior mutual understanding about which of the movies was good and Alice immediately knows the particular movie he saw. Also, she knows that he decided upfront to watch (only) that movie.
EDIT: last point, if they don't have a prior understanding about which movie is "good", Bob would have to have watched _all_ of them to be able to say "THE good one". (Because, if he only watched a subset, an unwatched movie could be better than those he watched and would make him change his mind about which one is "good").
> she has learned that Bob thinks that there is only one good movie in "those movies".
According to your explanation, she still doesn't know _which_ movie he saw. She knows that only one was good and that it was the movie he saw. But then you could just say "I watched good one".
> "I saw a good one"
.. here, he picked a movie at random and it happened to be good. Yay, "a" does make a difference! :)
This doesn't work in English, because both speakers are not assumed to have the same knowledge of context. The "the"/"a" distinguishes between something that describes and something that also determines.
Alice: Did you see any of those movies?
Bob: I saw the good one.
In this sentence, Bob is saying that "good one" can be figured out from context. In other words, Bob is saying that there is only one good movie in "those movies". Alice may or may not be aware of this from context, but she has learned that Bob thinks that there is only one good movie in "those movies".
Bob: I saw a good one.
In this sentence, Bob is saying that "good one" describes the movie he saw, but he is not saying that this determines which one he saw. He is not making claims about context.