To review: the Martian Zookeeper requires a Cow and a male and female Human, but ties are broken by whoever has more total Humans. So if player A is all set to win when they play the Goal, but B also meets the conditions, with more Humans, but they’re hidden in the Abduction Chamber, can player B reveal their Humans and win?
A: Player A wins. The Abduction Chamber can be accessed at any time, but it’s neither instantaneous nor retroactive. I might choose to set my Humans free during another player’s turn, when a change in the Goal or the Rules has altered the situation, but it does take a moment to open the cage and round them up.
In other words, if you’re set to win but you wouldn’t if my Humans were loose, I can’t stop you from winning by releasing my Humans. It’s too late, you’ve already won.