He can testify. He knows this. But he also knows that certain subjects can be brought into the case that he may not want that the judge announced at the beginning of the trial. And he knows a gag order doesn’t preclude him from testifying. What does he think his supporters are? Stupid?
No, not stupid. As Responsibility2nd said he was being facetious. Mocking the court “I’d love to testify but the judge will fine me or lock me up for speaking about the witnesses”. Or course, on the stand, the gag order does not apply. But the judge could be a jerk and sustain every objection to his testimony as irrelevant and try to prevent him from speaking his mind.