This is a fallacious appeal to authority. Credentialed economists are not the only people who can make claims about economics. If you can substantively debate her assertion then please do. Otherwise you are not contributing.
(I'm not familiar enough with Ms. Warren's background to have a substantiated opinion either way, though I certainly don't doubt her intelligence.)
BurningFrog is arguing that that is not correct because Senator Warren is not an authority in the first place.
You could still argue that she is correct; but (assuming BurningFrog's judgements on her qualifications), that arguement must be on the merits of her arguements, not her qualifications.
In it, Nokinside claimed that Warren is an expert in Economics due to her academic career. That's what I'm arguing against.
I of course agree that people from any background, including Warren's, can make claims about Economics that should be discussed on their merits, not dismissed (or accepted...) because of the speaker's academic background.