Simple, they put a contract extension on front of him and he signed it.
But more seriously, it seem to me like his mistake weren't enough to make fall out of grace at Arsenal. I guess their value his experience and forgive his performance against city because he might have no yet been fit. As for Luiz no doubt he feel like he hurt the club and need to make up for it. Thats really all I can see.
I don’t know if I got it right, but did D Luiz really sign a contract extension with Arsenal?
If so, how could this be?