Interesting question. I think the ED was probably animated after, or separate from the track. After would make sense due to the less amount of effort, but Tkmiz could have just had the layout of the ED already done. Entirely possible that that storyboard had a lot more animation to it and was just cut up or edited to fit the track.
Unlike Senko-san where its pretty obvious it was after the ED.
Actually, it could just similar, except the dreams are overlayed upon the sleepy senko.