Prostate cancer isn't that huge a deal unless it's already aggressively spread. And anti-androgens can stymie prostate cancer growth, so that makes me doubt it's prostate cancer without knowing anything more. He said it will impact every area of his career, and the only way it seems that could be the case with prostate cancer is if it's particularly aggressive and requires severe treatment that generally disrupts his bodily health.
I guess the trouble is what he means when he says it impacts his career. That could mean a brain tumor that will disrupt his entire life, or some illness either related to being a troon or being a man, which would impact his career in terms of public perception. From that perspective, he's still mostly pretending to be female so admitting to prostate cancer would have a disproportionate impact on his troon activism career.