Saying that something is only in someone's "head" is an idiom. The article doesn't assume that God has a physical head or brain, so that's a red herring.

If saying that brains are "in" minds appeals only to conceptual entailment (to one concept being logically or semantically "in" another), then metaphysical idealism amounts to mere word games or stipulations. You'd be saying the concept of consciousness entails the concept of brains.

Yet even if that were so (which it's not since naturalism is logically coherent), saying that the concept of mortal minds is contained in the concept of God's mind isn't the same as saying metaphysically that mortal minds are "in" God's mind--unless you want to say that God is just a concept. If not, your appeal to "conceptual space" won't make sense of the metaphysical idealist's talk of containment or pseudo-spatial relations.

By not addressing it in your comment, you seem to concede the point about metaphysical idealism's lack of distinguishing testable implications.