Yes! I had the same thoughts too
Aww... I was wondering why he didn't call his brother hyung despite how attached of a bro-con he is.. They should've made Yoohyun call him hyung.. It's more endearing, cute and fits his bro-con personality more
even outside of his broconness, korean culture (and asian culture in general tbh) puts a certain amount of pressure on people to be respectful to their elders. calling your older sibling by their name really isn't respectful at all. yoohyun would never disrespect his beloved older brother by calling him anything other than 'hyung', even if he weren't a psychotic bro con.
it really annoys me whenever i see yoohyun saying "yoojin" to refer to his brother directly. idk if the og raw manhwa is like that, but at least in the novel yoohyun would NEVER call him yoojin, only "hyung". yoohyun is too polite to call his older brother anything but that!