These are the sorts of things where the line between zero and practically zero gets blurry, so people feel the need to emphasize that it might not be zero. Like, the chances of me finding a winning lottery ticket on the street without buying one might not technically zero, but the odds are low enough that not only is it not going to be part of my financial plan, but I also don’t feel the need to justify why.
The odds of hyper drive aliens being on earth is zero. There might be an error bar on that number, but it doesn’t practically matter