His magic wont be nullified as August can't copy magic with specific requirements to use them. Its literally the reason he can't copy holder magic.
Why did August use a barrier to stop Laxus' L bolt instead of copying and neutralizing Laxus' DS magic? Because he can't.
Why didn't August copy Irene's slayer magic to boost his own attacks? Why didn't he copy Serena's magic the first time they met? Why did August use regular fire magic when regular fire magic is weaker to DS magic? Why didn't August just copy Acno's DS magic and neutralize his power so he would be at most half of his power?
He can copy the basis of caster magic but he can't copy the caster magic that needs specific requirements to use. All holder magic has specific requirements which is the reason he can't copy any of them.
August can copy the basis of Takeover but he can't copy Satan Soul or God Chronos Soul because he doesn't have the requirements to use them. DS has only two specific ways to use that magic and that's either by being enchanted by a Dragon or using a lacrima that most likely enchanted by a Dragon. DS magic can't be just be taught or everyone in FT would be DS lol.
August can't nullify magic with specific requirements to use them? Well, that has never been stated in the manga, this is your own interpretation. In the manga though, it is said that August can copy magic aside from Holder type, which DSM isnt.
Why didn't August copy Age Seal and end every battle before they even start? Because Age Seal is a holder type magic.
Wait, no it's not.
I mean if we're going to make a list of all the possible things August could have done but didn't do, this will never end. That's not proving anything. At best it makes it understandable that you're doubtful about it; but this is not enough to convince me.
Again, that's your own asumption. DSM doesn't need any particular requirement to me. It is no different than Fairy Law, who Cana stated was only accessible to FT members and which the tatoo enables, before it was revealed by Bluenote that it could very well be used by someone else. And August negated it.
DS magic can't be taught or everyone in FT would be DS? What? Then why isn't everyone wielding EVERY type of magic?
Why isn't everyone a high enchanter, or a God Slayer...?
This is not even a point. Anyway, i'm not going to convince you, and you're not going to convince me. Let's leave it as that.