Honestly, I don't know much about "asian culture" because, you know, Asia is not just one country. So "asian culture" is quite a vague generalization. But I'm okay with that. I am an asian btw even though a southeast one.
I get what Isayama is trying to say with that Eren-bird thing. It's the whole reincarnation theme, might be yes, might be not, definitely not literal. A bird is a symbolic representation for freedom, I get that.
I still don't like it. Because I honestly think we could get much better other ways to convey that Eren-bird symbolic implication than the one we get in 139 (please don't ask me how, I'm still looking for an alternative way).