People believe it because they know what it's like to get curved by one of these women. Women will curve men over some of the most stupid shyt, but income and height are two pieces men are especially sensitive about.
They're exagerrating but at the heart of it is hypergamy. A woman will curve a man who makes the same money as her or slightly less. She'll curve a man that is her height or slightly shorter. The goal is always a man who is superior. Superior in intellect, finances, emotional maturity, career accomplishments, physical fitness, cultural understanding, social status etc. Feminism has a major blindspot when it comes to addressing that issue. All things being equal, women still curve men on their level or lower when they know they can attract the attention(not commitment) of a high value man who has all the traits above.
The endless cycle these women find themselves on is due to the fact that they refuse to be satisfied with a man who is their equal or inferior in any way, and the men who are superior will never commit. Theyre willing to drop a regular dude when a high value man even gives the appearance of being interested in them. The problem is that they don't recognize or refuse to accept that the guy with options see the women who chase them as just some easy p*ssy until the woman who they really want(usually someone who just has a radiant attitude, very minimal baggage and a sense of consistent appreciation for herself and him) comes along.
The issue is that KS is telling women to be realistic and literally everything else is telling women that choosing a man who is your equal or less than you in some ways =settling for less than you're worth.