The Alien is not a deux ex machina as they already shown way even before Fortuna's chapters. In fact, similar situation already happened before with the spirit of the planet where it appear the first time Fortuna used spirit circle then stopped him when he was about to consumed the whole planet. It goes within the arc of Fortuna's character where he believed himself to be all knowing and capable of predicting fate and future. Mizukami wanted to show how such view is not only incredibly arrogant but also extremely selfish as there are infinite numbers of way which your actions, therefore future, could be altered (it also happened right before with the birth of the twins!). If you follow his works before, this is almost the same with Lucifer and The Biscuit Hammer where the villain is also arrogant and selfish knowledge hungry individual.