They are hardware enthusiasts on a tech forum so their idea of what is midrange is based on their hobby and wanting to justify the costliest GPUs they can buy. Its like saying just because Ford make the $500000 GT40,the mainstream Focus/Mondeo needs to scale up to the price to match the GT40. Casio make mainstream and midrange watches under £200,but also make handmade watches in Japan which cost as much as a Rolex. High end pricing does not have an effect on what is considered mainstream/midrange.
High end products have a much higher ceiling in price.
Even Nvidia disagrees with them - the GTX1660,GTX1660 Super and GTX1660TI are all under $300. So your analysis is correct - you can quite clearly see this in the Steam figures where the majority of GPUs are under $350,ie,the mainstream and midrange tiers. So under £300ish.
JPR and Mercury Research actually have figures for what is defined as midrange and enthusiast,and AMD and Nvidia use them both in their marketing slides:
https://www.jonpeddie.com/images/uploads/The_Balance_of_Power_in_Gaming_2019_-_V8.pdf
Midrange is now defined as $150~$350,mainstream as under $150 and high end/performance over $350. Nvidia and AMD actually use JPR/Mercury Research definitions in their own PR slides if you look carefully.