Says the white South African?

I watched a movie, 'Blood Diamond',
which is about diamond coming from conflict zones,
and here's a scene where a man and a woman meet for the first time.

W : The world is falling apart, and all we hear about is Blowjobgate.
M : When was the last time the world wasn't falling apart, huh?
W : A cynic. Why don't you sit down and make me miserable?
M : Danny Archer.
W : Maddy Bowen.
M : Pleased to meet you.
W : Nice to meet you.
M : American, huh?
W : Guilty.
M : Well, Americans usually are.
W : Says the white South African?
M : I'm from Rhodesia.

A pretty simple and easy conversation but this part.
Of what she said, 'Says the white South African',

What does exactly 'says' mean here?
From the context, I thought it's like,
"I guess you're a white South African.'

When you try to guess something,
do you native speakers sometimes use the term 'says', like,
'Says you are from S. Korea?'
'Says you're married?

I initially thought it's because she didn't speak standard English,
but her role in the movie is a reporter who's from the U.S.
,which means it's not very likely that she used some expressions
native speakers would find awkward or weird.

Beginning the sentence with 'Says' while trying to guess something.
Is that common?

I looked up the dictionary to learn the usage of 'says'
but there's nothing like that.

Could you please help me figure that out?