And every popular implementation shows arm + forearm + fist. [1] Giving only one part of that as the length is just wrong.
Even if it were called arm, I would argue that that should be shoulder to wrist, since that's what commonly called an arm. It's also the definition that Merriam Webster has. [2]
[0] https://unicode.org/emoji/charts/emoji-list.html#1f4aa