It doesn't say the sun's ray can nuke someone, it says that using a sun ray in Astarian results in a voice line.

Because, “spoilers” I guess, Asterion is the only one who says something there if it kills him. According to the posts I saw from the person who made the video on Twitter, other characters will die if it hits them, but none of them have dialogue.

so the first spoiler is what Astarian is

Someone pointed out that you can find that out before the the game even starts if you watch his origin video in the character creator.

a voice line is prompted by the interaction

You have got to be kidding.