It may well be. Prayers for Fred and his family.
Politically, revealing it without making an announcement about NOT running makes me believe he is running.
If he is going to run, he needs this information out in the public so people don’t think he’s hiding it. If he wasn’t going to run, he wouldn’t have to let people know.
If he wanted to use it as an excuse not to run, he would use it on the day he announced he was not running.
So it seems likely that, given that he isn’t announcing that he just learned it (he’s giving historical information), he is clearing the deck of the issue, to let it percolate through the news cycles and let people get settled about it before he’s a candidate.
It also gets him a news cycle, and a positive one at that, because almost nobody attacks a cancer survivor.