Here's what I checked out before I ask my question.

Not only- but also- has various versions like the below sentences.
▪ Labor is not merely a necessity, but (also) a pleasure.
▪ He gave me not only advice but money as well.

However, those versions have consistent meaning, that is "used to emphasize that something else is also true," which means the information with 'not only' is true and the one with "but (also)" is also true.

Here is the question sentence I came upon.

Not all authors trusted that the theater audience would automatically understand their plays in the intended manner. Thus, they repeatedly attempted to make it clear to their public that visiting the theater was not merely for the purpose of entertainment, but rather to draw lessons from the play offered onstage.

What I'm not sure is whether the information with 'not merely' is true or not.

In a dictionary, it says the below sentence, which opposes the information with "not." But it's not with "not merely" or whatsoever.

4 used to introduce an idea that is different or opposite to the idea that you have stated previously:
▪ The walls were not white, but rather a sort of dirty grey.

Can you give me an insight to this question?

Does "not merely" opposes the information with itself because of the existence of "rather" or agrees the information with itself because it is under the influence of "not only- but also- structure"?

Thanks for reading this question in advance.

I don't think my thread reads well, but that's the best I can do.
The writer has mismatched the correlatives, so the sentence isn't really logical. The writer should have chosen either 'not merely X...but also Y' or 'not X... but rather Y'.
Thanks, Mister Micawber.