I'm not necessarily saying Ulti's hardening > Luffy's hardening. But if Luffy's hardening was good enough, it should prevent such stuff.
But this is why I'm anti Zenkai boost. It's clear Luffy's hardening hasn't improved at all since FI
Oda's
"Haki gets better against strong opponents" schtick is just a plot tool he brings out whenever he thinks it's relevant.