the second sentence: could you please expand on that?

I'll try: any claim that a fundamental/terminal moral goal 'is good' reduces to a tautology on this view, because "good" doesn't have anything to it besides these goals. The speaker's definition of goodness makes every true claim of this kind true by definition. (Though the more practical statements involve inference. I started to say it must be all logical inference, realized EY could not possibly have said that, and confirmed that in fact he did not.)

I get that the function does not evaluate itself at all,

Though technically it may see the act of caring about goodness as good. So I have to qualify what I said before that way.

Why is this a feature?

Because if the function could look at the mechanical, causal steps it takes, and declare them perfectly reliable, it would lead to a flat self-contradiction by Lob's Theorem. The other way looks like a contradiction but isn't. (We think.)