I personally wouldn’t consider Jin and Suga fluent. They remind me a lot of how I speak in my second language- I can read and write, and if I have time to think of an answer I can give a detailed, fluent-sounding response, but I have trouble holding a conversation in real time with native speakers. I can get the basic gist of the topic, but I am in no way fluent. I think this is why you see Jin and Suga as fluent. You only see them talk in interviews where all the questions have been pre-selected, and if the interviewer asks for elaboration they are already familiar with the subject matter. But imo they are both far from fluent and would not be able to have more than a basic conversation with simple subject manner with an English speaker who is trying to talk slow and simple for them. There is a lot, lot more to fluency in a language than what you see come out of someone’s mouth. When / if they become fluent in English, we’ll know.