At the end of Night 4, Nate says that Mark helping them to catch Springtrap would provide proof that he wasn't lying to the police about the animatronics. In the Supercut's epilogue, MatPat says that the police found plenty of incriminating evidence on the security cameras. This likely included Springtrap's rampage, giving Mark the proof he needed and allowing him to go free (likely on a self-defense technicality, since he now had proof that he had reason to fear for his life), hence why that plot point is seemingly dropped.
