Mostly cuz of the wintery outfit & the crying... she's only ever cried in ballad PVs (HEAVEN, HANABI ~episode II~, rainy day, fated, etc)... The dancers have been in ballad videos before (Marionette comes to mind), so dancers doesn't necessarily mean upbeat dancing. They could also just be acting, too, not dancing. We dunno
But most signs point to ballad.
And in the message she said "Ishii" was the director, crying with her when watching the monitor.