It's an interesting trope, in my opinion. When you're faced with a dilemma of "letting this outside person, who has intimate knowledge of a very delicate situation, go and do their own thing to potentially ruin everything we've worked towards." What other way can you test them besides a test of their raw reaction?
If you tell them it's not real and a simulation, they'll know they can pretty much do whatever they want because none of it is real.
I mean the dude is sitting with the knowledge that there are humans capable of becoming prehistoric beasts from the past and now he knows aliens are real. Exposing all of this would literally change the world as they know it.
Doesn't change the fact that, from this child's perspective, he just experienced the very real danger of almost seeing his dearest friend hurt and suffering blunt force trauma from a metal bat. Doesn't matter if it amounted to nothing, he still experienced it.
Congrats, you proved he can be trusted. Unfortunately, it's very possible that he'll wake up in a cold sweat remembering that moment multiple times throughout his life. Also, if real danger comes up, will he even be able to believe what he's witnessing? Is it just another test?
This "It was all a test" or "It was all a joke" using severely traumatizing experiences trope is old as hell and always short sighted. Sure, almost no series actually bothers to give any stakes or consequences to the things, so it's mostly a moot point, but I'm one who has a tendency to empathize with all characters, and I if were him (or any of the hundreds of characters that go through this crap in manga), I know I would not be able to trust any of them. Being manipulated like that, to such an extreme degree, feels horrible, and I wouldn't be able to accept that "All the pain and fear you suffered that will probably haunt you for life was just a test" as of it was totally ok.
Edit: To be clear, I'm not angry or yelling at you for liking the trope. If you do, cool, I'm glad it has its audience. It's just one that, when you read as much manga as I do, you see it come up way too frequently, and it's always severely traumatizing events, usually done to children, that's severely underplayed for plot or comedy purposes, and it gets aggravating after a while. Please don't let my frustration keep you from enjoying it if you do.