Unlikely, if it was a security update then iPhone5C should have received it as well. iPhone5C didn't receive 10.3.4 update because its GPS hardware supposedly was an updated one than the iPhone5.
Also, Google Project Zero article(link in parent comment) gives the clear timeline of the attacks and that the exploits were updated to mitigate patches with every new OS updates; so even if 10.3.4 had security patches older devices still might be vulnerable to later attacks.
Also, Google Project Zero article(link in parent comment) gives the clear timeline of the attacks and that the exploits were updated to mitigate patches with every new OS updates; so even if 10.3.4 had security patches older devices still might be vulnerable to later attacks.