The human body naturally limits you to survive. Enma just breaks that limit.
Enma sucks up Haki into itself, it has nothing to do with the haki into Zoro’s OTHER SWORDS
KoH is activated when Zoro puts AN EQUAL AMOUNT of haki on ALL HIS SWORDS… Enma is demanding a specific amount of haki, which Zoro gives it… and then ZORO HIMSELF GIVES WADO AND SANDAI THE SAME AMOUNT OF HAKI THAT ENMA WANTS
Get it? Enma’s haki suck is unnatural in only the fact that it will keep sucking even to the point of death… Zoro is the only character in the story that has ever said that using haki can kill him… but the actual amount of haki that Enma wants isn’t a big deal in and of itself… Zoro triples Enma’s haki suck every times he uses KoH when he does it on his other swords…