Hi, part of the team here.
I suppose this probably deserves a little more clarification than the VisualArts tweet, so here goes:
We haven't been rejected from release on Steam.
The main issue here, as some have surmised, is that we are required to fill out a mature content description, and we did have two specific CGs pointed out to us during the initial review process as the reason. This would be the first Key game to receive a mature content description despite the game's all-ages nature.
We are still in discussions with Valve to see what our options are, but with VisualArts (and most likely Valve) entering holiday mode soon, this isn't likely to be resolved before the end of the year.
(Obligatory not VisualArts employee disclaimer)