I'm not convinced that that's true. All we know is that one vulnerability was a "buffer overflow" - pretty vague. If this were, for example, an overflow on the heap, which Grsecurity mitigations would even impact it? Improved ASLR? Mprotect restrictions? There are very few mitigations in the Grsecurity patches that even touch userspace in a way that isn't focused on kernel protection.

Maybe if it's a stack based overflow something like PAX_RANDUSTACK would have had some impact but it depends.

And in case this at all comes off as me thinking anything negative about Grsecurity, I assure you that's not the case. I proudly wear the "Grsecurity Cheerleader" badge that Spender threw my way over a decade ago.