Well no it’s pretty sound. Basically it means that most would not pay $500K for your house if they see others like it for sale for $450K. So your house can’t be worth more than alternatives readily available. Therefore it takes into account falling market price listings.
Although it seems that not every appraiser understands it or applies it appropriately though…that’s why we have other tools to look for listings ourselves.
It's called competition... no need to create more psedu-science to try and justify the process.
Houses are unique by their nature and are not fungible commodities... though they do break down to 5 or 6 variables that the market tends to value.
And even if you gave a dissertation on the value of say GOLD today, it would tell you very little for the value of gold in a month and absolutely nothing about the value of GOLD in 5 years.