The emoji isn't called "arm" though, it's "flexed biceps". Alternative names are: biceps | comic | flex | flexed biceps | muscle. [0]
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]
Just so you know, I'm fine with calling "arm" what is between shoulder and wrist.
I was just annoyed that GP seemed to diminish OP's efforts in building their page by nitpicking chosen lengths and terms, and I chose to enter the game rather than nothing.
Your comments are welcome and informative, thank you for that.
Dictionaries are descriptive and not normative, which confirms the common use of arm as "wrist to shoulder".
> Just so you know, I'm fine with calling "arm" what is between shoulder and wrist.
Great. Since that's actually what it means, in ordinary English. What you call "actually correct" "in anatomy" is professional sub-language and has no bearing on this forum. (Or any other, besides fora for professional anatomists.)
Edit: Sorry, replied to your first paragraph before even reading the rest and noticing that you actually agree... I was already slightly annoyed by your previous post, so wrote hastily. I apologize.
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.