Theoretically it's possible. Humans are prone to seeing nonexistent order in chaos — if you take five different-coloured batches of tiles and want to tile your wall randomly, you'll have to be quite careful in arranging it to look random — if you actually do it randomly, you'll end up with sequences and patterns in there that don't look properly random at all. And if you throw five coins in the air at the same time, sometimes you'll get five heads.
In this situation, obviously, shortlist selection is not just random. There are people making choices as to who to include, who will be involving their own biases and prejudices and aesthetic and sexual preferences, as well as all the powerful and biased factors influencing who gets to the stage of being available to be chosen.
I don't think it's impossible, though, that the five best artists this year (if we could define "best" objectively, which we can't) just happened to be men. What do we gain by arguing that it would be a literal impossibility for that to be the case? The arguments are no less strong if we concede that, even in a totally equal society, some years would have fewer or no females and some years fewer or no males.