override fun execute(args: List<Iota>, ctx: CastingContext): List<Iota> {
        val cond = args.getBool(0, argc)
        val t = args[1]
        val f = args[2]
        return listOf(if (cond) t else f)
    }

This is the impl, and this looks like it does what it says in the book, and I also tested it ... am I just brain farting

What about it is incorrect?

ah, nevermind, I read the book incorrectly; I was expecting it to be "keeps the second and discards the third; otherwise keeps the third and discards the second.", and read what's actually there as "keeps the second and discards the third; otherwise keeps the second and discards the third."
My bad 😅