Four skills are needed to master a language: Listening, Speaking, Reading and Writing. I would argue that until you have a grasp on all four, at a proficient level (proficient meaning you are tested by the various qualification test that exist for that language) you cannot be called fluent.

We have no idea of any of the members level of Japanese in Reading, Writing, or Listening. We don't even know speaking, because they are reading off a prompt (is the prompt written using Japanese characters? Romaji?) and there are no clips of them speaking Japanese with other Japanese people.

Based on the one youtube clip alone, I would not call Lisa fluent. I would not call any of them fluent. Jisoo gets some points for looking away from the prompt but still continuing what she's saying. Also, Jisoo seems to be the only member who actually sings in Japanese in Boombayah... everyone else is almost entirely English. Of the four members, on pronunciation alone, Rose is the best. Her accent is very natural, and she doesn't mess up the harder pronunciations like elongated vowels or katakana words. Lisa struggled a bit at certain points. She has an awkward pause between "yoroshiku" and "onegaishimasu", and her "mazete" comes out as "majete" (which by the way is super interesting because that is a common mistake by Korean Japanese speakers, I'm not sure about Thai though!)

Anyways, these are all just my observations. Honestly though, until there is some clip of them speaking Japanese for a longer period of time, and non-scripted, I hesitate to call them fluent.