It didn't cut his skull because he caught it with his teeth, stopping it from penetrating. Yes he's superhumanly strong, no ****. He ripped the prow off a ship and flattened people with it seconds beforehand.

Know what the chakram did cut though? His mouth. Because he's not resistant to cutting attacks. Pretty sure there's an SBS to confirm this as well, if it being repeatedly stated in the series that he's not, and the multiple occasions he's been nicked by blades don't count for some reason.

There's a reason Luffy typically DODGES cuts rather than blocking them.