The usage of だと and それなりに in this sentence

Question

I ran into this sentence and I'm a bit confused.

魔法の事についてだけはあいつもそれなりに真摯なのだと

I guess I'm more confused about the "da to" part. Is this supposed to mean "suppose to"? And so the sentence would be "When it comes to magic alone she's supposed to be earnest/sincere"? Or is there any other usage for it that I'm missing?

I'm also a bit unsure if それなりに has any important significance in this, like "she's supposed to be sincere in her own way"? I've read some examples of this expression being used and it never seemed to change anything meaningful so I'm a bit stumped.

I don't think the follow up sentence is important to the context but I'll leave it here anyway.

なぜそんな買いかぶりなどをしていたのか。

Answer 1

と

On the contrary, your following section is super important, so that I can know the と isn't at the end of the sentence but qualifies the next verb.

The whole clause before と is supposed to go to a verb phrase 買いかぶっていた "had been overrated", which is separated into two parts 買いかぶり + していた during the insertion of など, making the original form obscured. After all, the entire first clause is explaining how the speaker overrated her.

と as quotative case particle can qualify any verb that means mental activity to detail what's actually said, felt and thought.

それなりに

in her own way

No. "In her own way" is 彼女なりに. Here it is それなりに. Since それ "it" is too vague to tell actual things, それなり is more like an idiom as a whole: "so-so", "somewhat", "decent(ly)" etc.

Answer 2

In this sentence, you could replace それなりに with そこそこ and だと with だそう with little meaning changes.

それなりに is being used here as an intensifier. It depends a lot on context and speaker, but in this case it'd probably be somewhere in between とても and ある程度. In English, perhaps "quite or "somewhat" would be close approximations.

だと can be thought of just an abbreviation of だと言っていた. In this case, the one who said the sentence 魔法の事についてだけはあいつもそれなりに真摯 is most likely whoever あいつ is, but could be a third person as well. だと is just showing that the view/opinion that 魔法の事についてだけはあいつもそれなりに真摯 is not originally from the speaker.

As a side note, についてだけは shouldn't be taken in a literal way here. It is just for emphasis.

Putting that all together, the original sentence could be translated as:

(He said / I heard that) when it comes to magic, he is quite serious.

In the translation, the portion in parenthesis is equivalent to だと and "quite" to それなりに.