use of 単に....だけか in this sentence

Question

speaker secretly follows a bunch of thugs. And watches them enter a somewhat hidden cave while being in a cheerful mood.

不良たちが浮かれながら入っていったのは、海岸からかなり離れた場所にある洞窟だった。

入り口に朽ち木が傾いでかぶさっているせいで、よほど注意しなければ洞窟の存在そのものに気づかなかっただろう。

> ……なんだ。単に秘密のアジトを見つけて浮かれているだけか？

My issue is that

単に秘密のアジトを見つけて浮かれているか？

Simply finding a secret hideout and they are excited? (speaker thinking it's an overreaction)

or

秘密のアジトを見つけて浮かれているだけか？

Finding a secret hideout, and they are only exited? (speaker thinking it's an under reaction)

makes sense. but the actual sentence

……なんだ。単に秘密のアジトを見つけて浮かれているだけか？

why? (they) simply found a secret hideout, are they are only cheerful? (can't tell what point this is trying to make)

having both 単に and だけか in the same sentence doesn't make sense for me.

Thank you far any clarifications.

Answer 1

単に［秘密のアジトを見つけて浮かれている］だけか？

Treat 秘密のアジトを見つけて浮かれている as one set action. As this answer explains, 単に and だけ are often used together for emphasis. You can drop one of them without largely changing the meaning.

単に［秘密のアジトを見つけて浮かれている］のか？
［秘密のアジトを見つけて浮かれている］だけか？

They all mean "(I thought they were doing something different but) They're simply elated after finding a secret hideout?"

As an aside, なんだ in this context is an interjectory set phrase said when the speaker saw something and got disappointed. "So this is it?", "How disappointing", "Gosh", "Gee", etc.

If it were "秘密のアジトを見つけて、単に浮かれているだけか？", then it would mean something like "Are they merely excited even though they found a secret hideout?" Note the position of 単に is different.