The key is that the input needs to be comprehensible.
If you bootstrapped those people with a month or two of 1:1 lessons with a native speaker and then started them watching anime aimed at young kids with no subtitles, slowly moving them up to more complex examples, still without subtitles, they would end up learning to understand Japanese passably well after enough hours of it.
If you just dump them “countless hours” of complicated Japanese which they can ignore while reading English subtitles, then they’re going to end up learning a few words and phrases but not really becoming fluent.
If they started with poor English reading skill it’s not a completely useless way to practice that though. (Reading books would be better, but reading anime subtitles is better than nothing.)
If you bootstrapped those people with a month or two of 1:1 lessons with a native speaker and then started them watching anime aimed at young kids with no subtitles, slowly moving them up to more complex examples, still without subtitles, they would end up learning to understand Japanese passably well after enough hours of it.
If you just dump them “countless hours” of complicated Japanese which they can ignore while reading English subtitles, then they’re going to end up learning a few words and phrases but not really becoming fluent.
If they started with poor English reading skill it’s not a completely useless way to practice that though. (Reading books would be better, but reading anime subtitles is better than nothing.)