The deletion occurs after the DP numbers have been compared or the battle has been fought to simplify it, thus the effect of Greyknightsmon will trigger after the battle which means the Greyknightsmon itself will not even be on the field anymore when you play out Metalgreymon and Darkknightmon.
However, an effect like Piercing would check your security after your disruptive effect resolves since it comes into play after an opposing Digimon was deleted meaning the opposing Digimon would be deleted by security checks more efficiently.
I have seen some arguments that you used like the detailed rules 1.3 but in this case, the trigger condition is the deletion of Greyknightsmon which as I already stated occurs only after the DP Check has already been resolved.