And I'm not denying that's the case... however, I can still point out Hydra Venom fucks up Servants as badly as it does for Humans(well... less so, Humans would instantly die just inhaling the shit and it melts their internal organs, Servants will die slower but still die the same way) so why would Ainz be immune to that?
Why would he be immune to Hassan of Serenity's poison considering that shit can corrode metal on touch and that's before we get to her actual use of her Noble Phantasm?
Zabaniya (ザバーニーヤ, Zabānīya?) is the name of the Noble Phantasms of all the nineteen people who have taken the title of Hassan-i-Sabbah. Each one of the nineteen had to earn their title by proving their piety through obtaining a divine miracle bearing the name of ShayṭānWP that met the criteria...
typemoon.fandom.com
Like it would make some real sense if again, it was demonstrated how far his poison tolerance goes but like everything else ever stated by them in SB, it's just "Take their word for it".