How do you know that that's Sakuya (and even more: Chen) in the picture?

The pixiv post is tagged as "original", no chen or izayoi/sakuya taggings, not even touhou.
There is only this comment:
あ、マスターが咲夜さん似なのはサークル側の設定です。
As I don't speak Japanese, I guess (babelfish), that "master" looks similar to Sakuya.
So I think, that this original character (master ?) is supposed to look like Sakuya (similar to), but you can't tell that nekomata is Chen.

The comment basically admits to the Master looking like Sakuya...which would suggest that it's not her.