His "GP" or whatever that is, is actually enough to bring those two back so he doesn't really need to use the zero code but of course there's a desire to bring "everyone" back (￣∇￣") then again you could say he could use the code to bring everyone that he has met back but then again would that really be good ending??
Bet he would rather return and save as much as who he can (especially those two), correct his mistake, regrets and be stronger that his hyung then get the zero code again with "everyone" ╮(￣▽￣)╭
We can see that he aimed for the zero code to make the wish to bring them back, but as Midnight said, "they" are not clearly defined - brother, lover, comrades...
Also, that was only his wish at the moment close after their deaths. Maybe the wish he actually tried to make years later was different. After all, he described life in that world as hell and their deaths were probably not the only very traumatic things they had to go through. Being revived after all that is not necessarily a good thing.
Maybe he tried to spare them from these cruel experiences and wished to rescue them before that happened? Or maybe he even wished to spare them from coming to that world at all and live in peace unknowingly in their original world? And since that was not possible, he goes back intending not only to save their lives but also to prevent these traumatic things from happening to them to begin with.
Yall the zero code was supposed to be some “omnipotent power that grants even the most impossible wishes” right? How come it couldn’t bring back MC’s brother and lover? Like it had the amazing ability to turn back 10 years of time for the whole world but not bring back 2 people from the dead which should be easier in theory ┑(￣Д ￣)┍