My theory on this, from observing friends who are still Zanna fans and can't be persuaded otherwise, is that "middle class" doesn't mean what it once did. Middle class once meant nice cars, nice house, nice regular holidays. It meant only the man working while the woman looked after the house and could do hobbies and interests etc. You could walk into middle class by getting any slightly fancy job.
Now, middle class is stratified but we call it all middle class. People with traditional middle class jobs and income can no longer afford the middle class lifestyle. My husband and I were both super working class, got degrees, decent jobs and make more than our parents. But rent killed us, loan repayments are mad, and we haven't had a holiday in 5 years. We have crappy cars.
And a lot of our friends are like us. People who have lives where you'd call us middle class but can't afford the luxuries you once could.
Zanna is selling the middle class dream. A big house, nice cars, lots of holidays. She's appealing to middle class women to inspire them to buy "small" luxuries to emulate her lifestyle. She's saying hey buy this nice workout equipment and this nice gym gear to look like me- and that stuff isn't completely affordable, it's just most people have other responsibilities and can't spend like, £3k on a rowing machine, but she's encouraging you to think, hey it's not THAT much and I DESERVE it, so most people would put it on credit or drain their savings. For her it's nothing- IT'S FREE! So she doesn't need to talk about how much it cost, the realities of that, so her audience think it can't be too bad, it's normal.
Zanna is upper class. Firmly upper class. And people still want to strive for those things that our parents got to have, we're working harder than ever for less, and she presents an idea that you don't even have to work you just deserve these nice things and people fall for it.