Trump does this all the time because he's not trying to futilely court the media, he giving them the rope to hang themselves. He's never going to get fair media coverage, but by saying stuff like this he gets the media to show how deranged and untrustworthy they are.
You are right, and it is a tactic that has worked well for Trump, especially when talking about rivals, or political issues. This is much more sensitive and personal to a lot of regular citizens. Even if you believe this only a flu, or less than a flu, there may be hundreds or thousands of deaths before the election. When a commercial drops with the body count, and Trump saying it is a hoax over the top, it is gonna hurt.