There are two aspects to keep in mind here.
First, in the Greek, Peter had already used the word for honour
in 1 Peter 2:4 (with a preposition attached: precious
) to describe God’s assessment of the living stone
people had rejected. As God honours the living stone,
so God honours the living stones.
Second, the quote from Isaiah 28:16 had ended with the promise that those who believe will not be put to shame.
On the spectrum between shame and honour, Paul wants his readers to know that the Lord God is not putting those who believe in some neutral spot part way in between, but is crowning them with the very opposite of shame, that is, honour.
7 So the honor is for you who believe, but for those who do not believe, “The stone that the builders rejected has become the cornerstone,”