I am not the OP, but "cancer" is used as, and I quote, "(figuratively) Something damaging that spreads throughout something else". In that sense, the commenter

I am not the OP, but "cancer" is used as, and I quote, "(figuratively) Something damaging that spreads throughout something else". In that sense, the commenter could be implying, for example, that Apple products damage the electronic gadgets industry with their closed and anticompetitive policies, spreading silently (because users buy more of them to use the whole ecosystem).