Black Panther came out back in February and has even seen two more Marvel movies come out since then.
By securing this nomination, the other thing that Black Panther does is remind everybody else that it’s out there.
There’s a reason that the vast majority of “awards bait” gets released in the last quarter of the year.
People have better memories of the movies they’ve seen more recently, so a film betters its chances of nomination by being released closer to the end of the year.