I would say two things. One Superman has other powers besides strength. Two, his assertion is not true. They can only estimate how strong Superman is, but it's never been shown. We honestly don't know how strong or how powerful Superman can get. Stories like Superman Prime and the even the original movie should suggest as much to someone. Some even suggest Superman can and will become a god. However Superman's strength is not driven by anger.
Also note Superman does not need to continually be stronger than Hulk, just stronger than Hulk at that particular given time. Hulk has been knocked out before, or at least held in check long enough to revert back to Banner. Accomplishing either of these things would be considered a victory for the person in question. So if Superman can land a punch on a less mad Hulk, and knock him out, he wins. If he can hold him back, freeze his head with ice breath to prevent him from breathing, and he turns back into Banner he also wins.