Ok I looked at the article and I have to say it's a bit unfair to her in that the headline is "Ocasio-Cortez: The Upper-Middle Class 'Doesn't Exist Anymore in America" but the actual text says this:
Ocasio-Cortez said a more moderate upper-middle class no longer exists in America...
Underline added for emphasis. I don't know what she means by "a more moderate." But saying that "a more moderate upper middle class no longer exists" is different than saying that an upper middle class no longer exists.
From that point on all the other stuff is moot. The fact that the Urban institute found that the size of the "upper middle class" increased does not contradict what she said and the author of the article is incorrect in writing "Contrary to her statements...."
She did not say the upper middle class no longer exists. She said "a more moderate" upper middle class no longer exists.
That does not mean I think she knows what she's talking about. But it does mean that the author of the article drew an unfounded conclusion (she was contradicted by the Urban Institute).