Quoting TheRon directly for this one:
This episode refers to itself as “post-12” which is why I listed it as 12.5 instead of 13. It contains absolutely no new animation. The only thing new about it is a few lines of narration read by Corona’s seiyuu.
While there really isn’t an immediate need to dive into this recap episode right away, it could serve as a great little refresher to watch right before the next season starts. So I say, grab it now and save it for later. You’ll be glad you did.