If Im remembering correctly, the characters introduced in Imprisoned only showed up again during The Day of Black Sun and they were only in that episode for like 5 seconds. It doesnt feel like they were very important unless Im forgetting something. I forgot about Zuko getting Kataras necklace though.
Given how important The day of Black Sun is I'd say having a supporting role there more than pushes your introductory episode out of being filler. Furthermore Haru helps get Teo and The Duke out the the air temple when Azula attacks.