What defines mid-fi versus high-end?

The ladder: low fi, mid fi, hi fi, high end.

Mostly by price and not necessarily by brand.

Audiophile quality from hi fi and up.

a product that reproduced music with limited fidelity,

{I’m deliberately going to ignore the remainder of the quote for now to stay focused}

That’s a pretty good general definition, appreciating exactly what high fidelity means - the reproduction of what is on the medium (eg, a CD) with inaudible noise and uncolored by non-linear distortion, and a flat frequency response in the human hearing range (paraphrased from Wiki).

On this basis, I’m gunna be presumptuous and suggest that high end can then be mid-fi or it may be hi-fi.

Because high end (as I understand the term from people like Robert Harley and others) often does not conform anywhere close to that definition of high fidelity for reasons, but rather can often have limited fidelity.

I’ll leave a discussion of brands and money to those with abundant experience with either or both of those two virtues..and the remainder of the quote which I am not qualified to address without speculating.

The best I can do is an analogy with the video world: mid-fi is 720p and hi-fi is 2k and up. And that is still being simplistic. You still need to know how to set it up properly in a suitable environment. 
Money is just a part of it but you also need knowledge, lots of it. So be ready to hire a professional if you have the dough but not the know-how. Thinking it’s all about money is tantamount to going to a restaurant and ordering the most expensive dish because it’s going to be the testiest. 
We also need to stop comparing hi-fi to sport cars and mid-fi with SUVs. If I am going out of town with my wife, I would rather drive a Lexus RX350 F-sport with a Mark Levinson audio system than an uncomfortable, stiff Lamborghini. In fact. I would pick a luxury SUV anytime unless I am going to the race track with the boys.  
Finally, it’s not totally subjective. We know what we want: we want the picture in our TV to look like what we see when we look through the window and we want our hi-fi system to sound like what we heard in that concert hall. Is it attainable? You decide.  

Mid-Fi: Any component not worthy of being included in Jay's Audio Lab $500K system!