"So I guess a sufficiently advanced ETI would be indistinguishable from God, because He is God."
Your concept of God is extremely limited.
You misread me. I'm saying that God is the only sufficiently advanced ETI to be indistinguishable from God. He is, of course, more than an ETI.
It's a trick question (tautology), as I see it, based on the definition of the word "sufficiently."
The author could say that any ETI who can't convince you he's God is not sufficiently advanced, so the original assertion must be true (can't be false).