I don't really know whether this is supposed to be usami renko or just an original steampunk-y girl outfit. The commentary on this, or the other post, does not reveal any link with touhou or renko.
The reason she is apologizing is some guy complained that her pose in post #4549 is incorrect because Chisato never aimed with her arms extended and instead used something called center axis relock. However, another person points out that she does indeed aims with her arms extended in the anime.