Gaban believed that ACoC was the only way to defeat a Holy Knight and you cant kill them, and it seems he only started questioning that after Loki killed his father. This means that even Roger never managed to kill a Holy Knight during his time.
But realistically, couldn’t they have used Observation Haki to examine a Holy Knight’s body more closely? If there was something like a metal heart like sommers, they might have realized that destroying it could be the key. Zoro was able to figure something like that out.
you don’t actually need ACoC to defeat a Holy Knight. Instead, what’s really required is enough raw power and precision to destroy the metal heart that keeps them alive.