I already knew about what she said. Her words were merely a [...].
I checked Thesaurus but none of the synonyms seem like a metaphor/expression. Maybe go ahead?
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"an acknowledgement" or "an affirmation"
I already knew about what she said. Her words were merely an acknowledgement.
What's a metaphor/expression for “confirmation”?
American car-culture has created the idiom give somebody/something the green light (as opposed to a red light) based on our experience of waiting in our automobiles at traffic lights.
To give permission for something to happen; "She's waiting for her doctor to give her the green light to play in Saturday's game."
Usage notes: also used in the form give a green light: "The House of Representatives gave a green light to oil exploration off the East Coast."
The Free Dictionary green light
If you want a metaphor, perhaps icing on the cake
An attractive but inessential addition or enhancement
Oxford Dictionaries Online
We can say that her words corroborated what you already knew. They were a corroboration.