Not trolling aside.
The holy knights simply have to be on par or greater than the admirals overall.
Who else is going to execute them? You guys take a comment by Robin that's 20 years old, in a series that goes through endless retcons.
It's very likely these holy knights CAN kill the admirals.
Bruh, no one is executing no one.
CD's have that power drunk attitude that they think the can do whatever they want regardless the circumstances.
You think Akainu would let an Admiral be put to death?
Who whould kill them?
From across the manga, we saw that the Admirals were and are responsible as bodyguards for the CD's, even Kizaru is protecting Saturn.
Look.
Now what happens to the CD's, if the power they wield becomes against them?
Would they command another Admiral to hunt other Admirals?
That would mean Akainu, with the low resources he has at the moment could potentially lose 2 Admirals.
The Gorosei stressed about one single Admiral being lost and deemed his strenght has huge.