Looks like I unintentionally called it.
Makes sense now when you think about it. Zeno can basically do White and Green dragon abilities under the condition he's damaged. (Can't remember if he was able to mimic the Blue dragon)
Plus effective immortality.
I imagine the Yellow dragon did it this way to hangout with the Crimson dragon more closely for his human lifespan, rather than watching from the heavens.
As I read it, I thought the same thing. When we were introduced to Zeno's abilities as a yellow dragon, I thought that for some reason he had more than one.
My idea basically came from the fact that Zeno has to hurt himself in order to act, but that helps his skin toughen and become stronger. If you think about it, that's pretty much correlative with the rest of the abilities, who had at least one part of their body like a dragon's. So, besides becoming stronger with injuries and having the ability to heal quickly like the rest, why was Zeno the only one who couldn't die? No matter what kind of injury he had on his human body, he would still live.
Right now, I probably understand that it was surely the work of the same yellow dragon, who wished to accompany the crimson dragon as closely as possible. Perhaps, like Yona and the previous Crimson King, the vow the yellow dragon had with Zeno was modified over time during the first generation. That would explain why, despite all dragons swearing the same thing, only the yellow dragon had such a notable distinction as being blessed/punished with immortality.
Or maybe I’m reading too much into it, who knows!