This is not a fan film. Except it is. There are three fans of Joss Whedon's (more so now, after he contributed substantially to their Kickstarter ) performing the theme to Whedon's Firefly. So it is a fannish effort, even though they are all cast members of The Flash.
It's good. It really is. Apparently the Flash set is one of the happiest places to work on TV these days (and why wouldn't it be? They have a great cast doing great things on a weekly basis.)