It's not just talking shit, Leffen also went after Hax's controller, The B0XX. The video Sprate Header posted goes into detail about the situation starting at 1:13:40
Basically, Hax designed an ergonomic controller so that he could still play Melee despite his injuries. This lead to controversies, especially concerning whether The B0XX should be allowed at tournaments. Leffen tried to ruin Hax's project not only by strongly campaigning against legalizing the controller for tournaments (claiming it would be too OP), but also crticized the controller itself, stating that it's outdated, or that the build quality is bad. He then went on to promote Frame 1, which was essentially the same project, managed by people who earlier worked on The B0XX itself (This led to a legal case started by Hax), and suddenly he no longer had an issue with having ergonomic controllers allowed at tournaments. So even though Hax went too far with Evidence.zip 2, it's obvious that the Swedish hapa consistently tried to screw him over, even after Hax's numerous attempts at atonement for his past meltdowns.