If this guy really has the source code of both 5 & 6 this obviously will add more development time. This one of the biggest games specially due to its online functionality, if bad actors have code related to that it’s no bueno.
I don't buy it. They're not going to re-engineer their netcode just because a development version of it got leaked. It will probably get rewritten anyway in the normal course of development. But if they were counting on an obscure protocol keeping hackers out of their game, they were always going to lose that advantage within hours to weeks of the official launch anyway.
