Regarding chapter - I did not expect we would see mu bleed. I did follow the spoilers this week, (i skipped the week of mu arrival, mostly because i wanted to enjoy that reveal), and knew the loki is going to go all out, and yet not going to win, but i did not expect mu to bleed at any point. this definitely puts him in finite power range. though his injuries healed pretty quickly, the fact that he can be injured, he certainlly is not immortal in this current time and position.
the theory that i did have, and was much improved by a comment i read on reddit is that (well it assumes imu has had the op-op no mi (law's fruit) immortality operation) the operation room where law can do anything is the only place where mu is immortal, it is like he stops aging in that specific room. whenever he leaves that room, he starts to age (and presumably as a curse, must age very quickly). considering he has "the devil fruit", he is still powerful (to put it extremely lightly), but not infinite life span. This is also why gorosei (5 elders) were concerned of mu leaving the castle - the flower room is the operation room, where life can blossom.
That theory would also be great to depict the importance of this fight - imu has left castle for first time in hundreds of years, because this is the first fight he has considered worth leaving his immortality for - cause joyboy and elbaph are back. Another thing that I believe is elbaph (as in, the god of war) was either the army head of the ancient kingdom in joyboy's era, or even before, just the god of war in one piece pantheon, like nika is sun god.
The peak piece just simply does not stop!