It's not like that Han Kang doesn't speak English and Smith butchered her works in secrecy. Han Kang has even explicitly supported the English translation so the "controversy" should be considered ungrounded.
The flip side is that she might not want to criticize the translator (and translations) that brought her the biggest amount of success & fame. Plus, it's possible that the author herself is more comfortable with the amount of "deviation" than some readers and that's a personal preference thing at the end of the day.
It's definitely possible and happened many times for other cases, but she actively worked with the translator in this case and had a good understanding of the resulting English text. Quoting a recent interview [1]:
스미스는 소설을 번역하면서 한강 작가와 계속 의견을 주고 받았다고 전했다. 특히 소설 마지막 부분에 주인공 영혜의 언니 인혜가 한 말, “꿈속에선, 꿈이 전부인 것 같잖아. 하지만 깨고 나면 그게 전부가 아닌 거란 걸 알지…(후략)” 번역을 두고 가장 오랜 시간 의견을 주고 받았다. 스미스는 이를 “surely the dream isn’t all there is?…”로 번역했다.
“‘surely the dream isn’t all there is?’ 하고 영혜에게 말하는 대목에서 (한강) 작가는 인혜의 확신 없는 머뭇거림이 영어권 독자들에게 전달되지 않을 것을 우려했고, 이에 나는 영어의 ‘surely’란 단어가 어째서 확신을 의미하기보다는 오히려 화자가 스스로를 설득하고자 노력하는 인상을 주기 마련인지 설명해야 했다.”
Or in my rough translation:
Smith stated that she kept discussing with Han Kang over the course of translation process. It particularly took the longest amount of time to settle on the translation of words from Inhye, an elder sister of Younghye the protagonist; she translated that as "surely the dream isn’t all there is? [...]".
"Han Kang worried that English readers might not get a hint of Inhye's inconclusive hesitation from that line, so I had to explain her how 'surely' in English rather often conveys sort of self-delusion instead of confidence."
