First of all, if Matsui actually introduces aliens into the series, I’m going to storm to Japan, locate his home, pound on the door until he opens it, and then scream “WHAT HAVE YOU DONE?!” in his face in Japanese.
And then Matsui (from what I could pull from his notes and interviews his personality is quite playful) would probably laugh his ass off and say “I did my job.”
Secondly, I don’t actually remember Korosensei ever saying that he would actively protect his students. When Karma asked if a teacher would risk his life to save a student, he agreed. But not because he would do it himself; it was because that protecting students is a teacher’s duty. If it were somebody outside of the class, I honestly don’t know if he would bother.
He would never harm them, and if they were put in some sort of mortal danger then he would fly to their rescue at Mach 20, but other than that he seems to be content with watching over them mature in the face of hardships. In a way he’s kind of like Reborn from Katekyo Hitman Reborn! He puts his students in dangerous situations (like the second Takaoka arc and Civil War arc) so that they can actively grow. He’s a teacher, not their guardian.
A teacher is not there to hold a student’s hand, but to guide them from twenty paces ahead through foreign terrain. He can warn them of the upcoming dangers, or explain how to handle a particular obstacle, but he shouldn’t lend a hand until a student shows true signs of struggle. And even then he has to wait to see if the student can handle it on their own.
Besides, Korosensei has faith in his students; he’s confident that they can overcome the adversity like they have many times before.