I think he heard some philosophy stuff out of context or over simplified, and was knee deep in inferences when he tweeted that. Something like:
The only thing we know with certainty that we exist.
So we don’t know if anything, including what we perceive, is real.
If our perception could be fake, we can’t even know that our sensory organs, like eyes, are real.
However, in my perceived life, I can look at myself in a mirror, and see my own body and my own eyes.
If my perception isn’t real, and my eyes aren’t real, how can I see them in that mirror?
If our eyes aren’t real, we wouldn’t be able to see them in a mirror for… some reason.
But my name is Jaden Smith, and I can’t understand that a fake reality could trick me into thinking I see my own eyes, so I think being able to see my eyes in a mirror means my eyes must be real, the mirror must be real, and the physical world exists with certainty. I disproved solipsism guys!
In all seriousness, I don’t think he believed his eyes weren’t real, he was asking a rhetorical question that he thought proved something deep. However, because he was a dumb teenager, he thought he could figure out something no one in history ever considered, and was thus making some grand contribution to society.