If you look at the way the elbow hinges (quite brilliantly, really), the arm won't get "longer" when straightened out. The forearm swings attached at the front of the elbow which stays in place. The elbow is inside of the forearm so when bent there isn't an unsightly gap. I used that same design on Bruderstator because it was so elegant.
I understand what you're getting at (and it is pretty clever), but it doesn't change how absurdly long the arms are. Those proportions are something I would absolutely expect jinbao to adjust. They love fixing/upgrading the figures they knock-up, and that one is too obvious.
There's really no way to fix the issue as long as everyone insists on the bots and alt modes all being EXACTLY - down to the millimeter - the same size. There's no way to fix it without having the upper arms hiked up way high. There's no way to fix it without having tiny forearms. And extra long thighs don't change gorilla arms.
When you have enormous upper arms (because the alt modes have to be the identical size of the other alt modes), this forces the forearm lower. When you have a proportionately large forearm, that exaggerates its length even more. The designers could move the entire arm up, but then that looks weird.
I feel bad for designers. They could fix the issue, but then everyone will complain about smaller arm bots. Toyworld almost had it with their partsforming (and people complain about that as well) crotch/thighs adding length, but they went and made crazy long elbows for.. who knows why. They could've designed a better elbow that wasn't so long. Maketoys also came close, but Giant has tiny forearms.
I was able to get perfect proportions only because I didn't have to worry about individual bot/alt mode sizes. I could have smaller arm vehicles that have the thumbs fall right in line with the crotch. I could have shorter elbows that didn't have people crying "But they don't bend 180°!!" These 3rd parties don't have those luxuries.
As long as the fan base insists on identically sized alts/bots, the proportions are going to be wonky without add-ons to hide impossibilities.