Earlier quoted context omitted.
Isn't that true of any assessment? With the ordinary property tax, you have to assess both the land value and the improvements. (There are of course ways of taxing property that don't involve that kind of outside assessment, but they're not commonly used.)
None of that changes the fact that there is no inherent value to land (improved or unimproved). Different people value the same piece of land (or any good) differently. This is just reality, regardless of whether someone is appointed to imagine a price for the purposes of taxation.
Valuations in currency are an abstraction for how we deal with this really tricky and intractable problem. But like all abstractions, it's leaky. A $100 bill has no "inherent" value yet it's value is very precisely known.