Within the context of Christianity, the best way to show the difference in kind is consider fallen Earth.
In transcedent account, fallen Earth is considered completely overcome. Reality has stepped to a transcedent level. Not merely ending or replacement of the fallen, such as a immoral state our world being replaced, but rather the complete undoing of the fallen. Almost like it never happened in the first place. More or less, the overcoming of sin.
Nietzsche's point is this is an illusion. Since life and values are talking immanent, there can only be existence. There can be no stepping to a transcedent level. We can pose whatever afterlife we want, whatever deity we want, it will will only be more existence. Fallen Earth cannot be overcome. Evil states may be destroyed or replaced, but they will never gain the "overcomed" status. Only Earth (Reality) has meaning and thus will always be the case, no matter how much is destroyed or what changes.
You're right that this has similarity to some aspects of Christianity. It can be seen as an extension of the Christian rejection of sacrifice to achieve meaning. Just as the Christian argues no amount of property, money or sacrifice will return you to meaning, Nietzsche is extending it to the
entirety of sacrifice.
Not even the sacrifice of the Son of God can return as to meaning. If are to think in those terms, we are no better (in the context of Nietzsche's critique) than the man who thinks paying the temple the most money will save him. We are no better than the man who thinks going out and killing another will save him.
In any of those cases, we envision that the sacrifice of something or someone else will return us to meaning, will make it as if our failings had never been, taking us up into the transcendent realm.
Meaning is infinite in an immanent context. There can be no transcedent answer. If I sin, nothing can undo what I've done, the people I've harmed, the difference it made to what happens in the world. God could sacrifice 10000 sons and I would have had the same terrible impact. Sacrifice does absolutely nothing with respect to undoing the meaning of my sin. All it can do is tick a bureaucractic requirement of God (or man, if we talk about having the most money, status, etc., which supposedly required to have meaning).
Meaning can only be given in life and affirmation. I'm stuck with sins past (as it was my life). The best that can be done is live without sinning into the future (as it would be my life). I am never without meaning. There is nothing to be "saved" in a transcedent sense. I could be "saved" from a terrible life, one full of suffering or unpleasantness, but this would just be a change in my living circumstances.
Thus, God is dead.
(Or if you prefer, God was never alive or acting because God is beyond such things, an infinite of meaning we cannot be separated from, even in our darkest hours).