Earlier quoted context omitted.
Arm: 30cm. I think my arm is bigger than that. And fire: 40cm. This seems somewhat arbitrary lol
Let's play picky: In anatomy, the arm is the part between the elbow and the shoulder. Below the elbow is the forearm. With that in mind, 30cm for the arm is definitively a sound measure... and the name of the emoji is wrong. On every phone, keyboard and app. tl;dr; what is commonly called arm is in fact arm + forearm. Don't blame the author for being correct.
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