Wording is very important and the way the entire paragraph is worded suggests that all these arms are is nothing but organic prosthetics.
Again, the paragraph refers to CINDER'S GRIMM ARM GRAFT, not the fucking arms that Salem summoned. Literally read the first goddamn sentence: "There is also an as of yet-unknown method for grafting Grimm anatomy to a human body, as seen with Cinder Fall's left arm."
This, exactly this.
It is not about Salem's arms, that paragraph, but Cinder's grimm Arm Graft.