Holy knights are battle oriented celestial dragons who work for Imu.
I doubt everyone who will fight them will have AdCoC
So it looks like it's more related to knowledge in how to affect
With that said, Gaban affecting them still doesn't nullify their immortality(Sommers is still alive), so most likely, they'd just be disabled in their battles without actually being killed(there are rare death cases in One Piece anyway)
Most likely the only way to completely remove the immortality would be to affect Imu directly(probably with the reverse process of the immortal surgery of the ope ope no mi, likely reason he is immortal in the first place)