I've explained this before. Luffy's CoA hardening is still
shit.
https://worstgen.alwaysdata.net/forum/threads/unpopular-opinions.150/page-61#post-199990
I said Law and Zoro have stronger Haki than Luffy in DR. This is why I never rated Cracker's Haki as Top Tier and I said that even Vergo might break Cracker's Haki and definitely Zoro will. Luffy has always gotten by on brute force.
So no it's no surprise that Ulti made his head hurt, when
Hody sliced through his Haki with his teeth. I also explained, that Katakuri will have stronger hardening than Luffy even in Wano.
The reason being is that
penetration Haki is not hardening. Hardening is Buso: Koka. ":" means Oda has different models. It's a different category of CoA altogether. So Luffy has Color of Arms: Penetration or whatever the fuck Oda wants to call it.
But yeah, base Luffy can still fodderize the fuck out of Ulti with penetration Haki without even using G2/G3 and it's not even close. But Oda wants low IQ Luffy at the beginning of arcs.