Can anyone explain what ないっちゃないんだけど means in this sentence?

Question

I'm reading the first novel in the Library Wars series at the moment, and using an English translation I found online to help out when I don't understand something. But in this case the translation hasn't cleared things up for me. The original Japanese is this:

堂上も大人げないっちゃないんだけどね (堂上 is a person)
Dojo can be pretty childish too.

I am confused about how the っちゃない is functioning in this sentence. I know っちゃ can be an abbreviation for ては, but in that case wouldn't it mean 'Dojo is not being childish either'? It doesn't seem the same as when じゃない is used in a positive way, because of the んだけど, which seems to emphasise the negative sense of ない.

In the context, either translation is possible, although neither seem to make perfect sense. The speaker was explaining to the main character that Dojo is only hard on her because he has high expectations of her (i.e. he is not just being childish when he is mean to her), but the main character's response, that prompted this remark, was very stubborn (i.e. perhaps she is being childish and the speaker could be trying to make her feel better about herself by implying she's not the only one).

Answer 1

Xと言えばX, where X is any predicate, is a way to reluctantly admit X.

• 3日でできると言えばできます。 I can do it in 3 days if you insist (but I don't want to do so)
• その花は赤いと言えば赤い。 You may say the flower is red (but normally, it's not red)
• そのプランは可能と言えば可能だ。 If I must choose between possible and impossible, it's possible.

And っちゃ is a very colloquial contraction for と言えば (maybe this contraction is found exclusively in this pattern)

• 3日でできるっちゃできます。
• その花は赤いっちゃ赤い。
• そのプランは可能っちゃ可能だ。

堂上も大人げないっちゃないんだけどね is the same as 堂上も大人げないっちゃ大人げないんだけどね, and means something like "Admittedly, Dojo is also childish", "I hate to say this but Dojo is indeed a bit childish, too."